first of all, there's another thread discussing this very subject RIGHT NEXT TO THIS THREAD, and it has the answer.
second of all, both of these threads have nothing to do with the discussion of tremulous and should be in troubleshooting.
third of all, ~/.tremulous/base/screenshots on *nix C:\documents and settings\your user name here\application data\tremulous\base\screenshots (or local settings\application data depending on how recent client you're using is) on windows ~/Library/Application\ Support/Tremulous/base/screenshots on mac, and base needs to be replaced with the mod you were playing at the time you took the screenshot if you were playing a mod.