Taking a screenshot is surprisingly a lot easier than you might suspect. There is a key on your keyboard (picture courtesy of Jremboul ~Pae) that says Print Screen on it (usually abbreviated as Print Scrn or PrntScrn depending on the keyboard used). Its on the upper right corner or your keyboard right next to the F1, F2, ....F12 buttons and right above the Insert and Delete keys.
1. When you want to take a screenshot, capture the image you want by pressing the Print Screen button on your keyboard.
2. The picture is now copied. Now go to MSPaint or any picture editing program you have (ex. Photoshop) and press the Ctrl button and V (Ctrl-V) at the same time. This will allow you to paste the image you just captured. You might get a pop-up message saying "The image on the clipboard is larger than the bitmap. Would you like the bitmap enlarged?", just click yes to that.
3. Now you can edit and save your picture to your hard drive.
This is a bit more complex since the Print Screen button does not work they way it should. If you are using KDE the above directions will work out. Instead of MSPaint you can use GIMP. For the rest of the linux users it requires a bit more work. Since there is no button that helps you do it you have to have a program that does it for you, I suggest GIMP since it will help you to capture and edit/save your screen shot. Unfortunately you have to know you are going to take a screenshot before you do it. If you see something cool happening on screen you will never have enough time to open GIMP and take a screenshot of it.
1. Open GIMP2.0, if you do not have it go to www.freshmeat.net and type GIMP and search for the project.
2. Click on File -> Acquire -> Screenshot. Now you have certain options you can either acquire a screenshot of the entire screen or a specific window making your life a bit easier so that you will not have to edit out any other windows you have on screen. The number of seconds delay is usefull if you want to take a screenshot of your entire screen but do not want certain programs to show, like GIMP.
3. Right after you acquire a screenshot a new window will be opened with the image there. You can edit/save as you see fit.
There are 2 ways to get a screen shot:
1. Press the keys Shift, the Apple symbol, Space, and the number 4(in this order just to be sure). Although this is 3 extra buttons in order to take a screenshot rather then the 1 Print Screen it's still handy. The picture appears on the desktop.
2. Use the Application Grab-if you need help locating it go to Finder an type in the search spot the word Grab and it should come up(icon has scissors with a window). There is no window that will pop up for Grab but in the top right hand corner there will be the Apple sign, Capture, File, Edit...then you will see under Capture if you click it Timed, Slection and Window. I usually use Selection which is like what it says: you kind of like crop the stuff you want to take a picture of. Window is wehre you click one of your windows and it takes a picture of that. Timed should be self-explanatory, but if you must know you just set how many seconds you want the Grab to wait to take a picture. The picue appeears on the desktop also.
Additional Mac information:
(Courtesy of JaredHawthorne)
Taking a screenshot in Mac OS X is Shift + Apple Key + 3. This immediately saves a screenshot to the user's desktop. The user can optionally cause the cursor to turn into cross-hairs allowing the user to select a specific region for capture by pressing Shift + Apple key + 4. They would then click and drag the cursor to select an area (like using the rectangular marquee tool in Photoshop). When they release the mouse button, the selected area is saved to the desktop. In either case, the space bar is not required.
Additional info players might need to know: In Mac OS X version 10.3 and earlier, screenshots are saved in Adobe PDF format. They can be changed to jpeg by converting with Photoshop, GIMP, or simply by opening the PDF in the "Preview" application and clicking File > Save As... and choosing "jpeg" in the "Format" pop-up menu in the resultant save dialog. In Mac OS version 10.4 and later, screenshots are in png format, easily converted to jpeg as above. Users may change the keyboard shortcut in the "Keyboard and Mouse" preference pane in System Preferences to something shorter if they wish, such as F1 or F2, et cetera.
Kat edit: *Extra info thanks to MarcMagus: GNOME users may find they can easily take a screenshot by simply pressing the Print Screen key. After a short delay, a dialog should pop up with a miniature image of the screen asking what file you want to save to. You can also hold the Meta (often bound to Alt) key while pressing Print Screen to save an image of the window with focus. I know this works in GNOME 2.10.0 with Metacity, I'm not sure if it's a feature of the Desktop Environment or the Window Manager.
Posting an image
There are two cases here, you either have the image you want to post on your computer or you found a cool image online. If you want to post an image you found online skip to step #5 , otherwise keep on reading.
1. Your picture will have to be saved as a .jpg or .gif file. You might also want to make your picture smaller making it a smaller file, which in turn will make the picture load faster. Any imaging editing program will allow you to do this.
2. Now you need a web host. Basically in order for people online to be able to view it you will have to put it online first. A simple search in google for "free web host" will give you tons of results. Make sure you select one that allows remote linking. (if you try linking on an image and you get some kind of error saying "remote linking prohibited" it means that you should find a different host that does). A good site for hosting is http://www.imageshack.us/ or http://www.photobucket.com/
3. Create an account at the web hosting site. You will have to upload your picture from your computer to your web host. Most sites either allow FTP access and some of them have an Uploading program. Read their FAQ to figure out what you should use. If you are using imageshack then all you have to do is click on the upload button, find the picture you want to upload and upload it.
4. Now that the picture is uploaded you will need to write down its web address or copy the address so you can paste it later. Lets say that your image link is http://www.battleon.com/Images/AdventureQuest-header5.gif
5. Posting your picture to the forum is the easiest step you can do it two ways. You can press the IMAGE button, when you are posting or replying to a topic. You get a series of grey boxes, B, I, U, S...image, all of them are PGP codes for the forum, "image" is one of them. The other way is actually typing the PGP command. Either way the end result should look like this:
Note: Being able to post images is a nice feature, but do not overuse it. Images tend to slow down posts considerably. Internet etiquette wise the best thing you can do is upload your picture and when posting in the forums give the link to the picture instead of displaying the picture. That way the post will load faster and whoever wants to see the picture will be able to.
Making Signatures with a Link
So now you have a picture and you are going to use as your signature but you also want people to click on it. Its a lot easier you might think.
Lets say I have a picture at http://img66.exs.cx/img66/1517/niko.jpg , as mentioned above all I need to do to display a picture is use the image tags. Now you have to know the link you want to use when people click on that picture. In my case its my guide, its address is http://forums2.battleon.com/f/tm.asp?m=5118. In order to have a link I need to use the link or url tag. Combining both we get
Notice that you use the url tag first and within it you use the image tag. A general example of it should look like this
[url="link that you want goes here"][image]picture that you want goes here[/image][/url]
Note the url=" ", the quotation marks are important.
Also check out this guide for information about all the other commands you can use.
Thanks to Niko for the original guide text.