This HOWTO is taken from an email by Paul Frields and could use some editing - if you can improve this page in any way (especially if you're the first one to use it), please do!
Set up a brand-new account on an up-to-date Rawhide box. Doesn't matter what the account name is, but I set the "real name" to "Fedora User" to be non-gender-specific.
Log in as that user, and set the display resolution to something fairly low, like 800x600 or 1024x768 using System > Preferences > Display.
All the other settings will be system defaults at that point, which is exactly what you want for accurate screenshots. Unless you're demonstrating how to change a default, you should use a default, so that the resulting shot looks virtually identical to what the reader sees.
There's a reference for formal docs here: http://docs.fedoraproject.org/documentation-guide/en_US/sn-screenshots.html
...but that's really for formal docs in DocBook XML and not wiki-based stuff. I don't know if there's a recommended maximum size for shots in the wiki, but my guess is that sticking to a 500px maximum width on the wiki page should be best. You don't have to resize the image yourself, I believe you can use the wiki to do it automatically like this:
[[Image:my_new_picture.png | 500px]]
The picture should automatically link to the image page with the full size picture available if needed by the reader.
If you need help with anything, please feel free to ask on the list or come by IRC Freenode at #fedora-mktg.