Using Vista I can't find my screenshots and I have tried many options including using the search function.
C:\Users\USERNAME\AppData\Roaming\Tremulous\Base\screenshots
Make sure you have Show Hidden Files turned on. You can toggle this by going Control Panel > Classic View > Folder Options > View.
Mine is in C:\Users\USERNAME\AppData\Local\Tremulous\Base\screenshots
:-\
found it and it was in the local.