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 or to the appropriate release. (e.g. if you're doing screenshots for Fedora 37 Beta, make sure you have a Fedora 37 Beta build or something as close as possible.) It 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 1280x1024 or 1440x900, unless the need is for higher resolution screenshots.
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.
In GNOME, you can take screenshots with the Print Screen key — they'll end up in the ~Pictures folder — or you can use the Screenshot tool. (Hit the overview key, start typing screenshot, hit enter.) This tool can save the mouse cursor or screenshot individual windows or a selected area, and it lets you choose where to save.
In any case, screenshots should be PNG files, unless a very large and decorated desktop makes the resulting file many megabytes. In that case, use high-quality JPEG, but be aware that JPEG is often terrible at compressing high-contrast detail like that found around text on a screen, so make sure the quality is set high enough that artifacts are not visible.
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 marketing mailing list or come by IRC Freenode at #fedora-mktg.