Screenshots are an essential part of any software documentation.
For PC/Windows users, the PRT SCR key is what you hit to take a snapshot of the screen. This is located somehwhere near or above the Backspace key (You probably don't need to press shift). Once you hit that key, the screenshot will be copied into your clipboard, ready to paste into MSPaint (the default windows program) or any other graphics editor of your choice.
Style Guide for screenshots.
Filenames and formats
1. All screenshots should be saved and uploaded as a (pick yer poison) file.
2. The filename for the screenshots should start with the pagename of the page it is intended for (or the first 20 odd characters anyway) using dashes or underscores for spaces.
Cropping and Sizing
1. Whatever it is that you are trying to depict - show the whole thing / table / module / screen.
Alternate Suggestion
If it's something that's very large, like the admin menu, then showing the whole thing is probably not necessary. In some cases, (quiz creation) showing the whole page can create huge images. Do use common sense and be ready to trim things up if necessary
2. Don't provide a zoom view unless you have already shown the whole page.
Alternate suggestion
However, you may want to zoom if you're showing the smaller part of a whole, such as the left menus
3. Leave your Browser toolbars and desktop out unless that is part of the thing being displayed.
4. For full size screeshots resize them to 75% of the original so they fit gracefully on the tiki page without making all the text illegible.
5. Don't blow them up unless necessary (for reasons i can't imagine).
6. When you are showing a screen shot of a form, show the form with all the necessary information filled in. This will help the user visualize how the form should be used.
Highlighting and emphasis.
1. Use the paintbrush with and set the paint color to red.
2. circle (by hand if you can do it neatly) whatever it is you are emphazising.