If you are using Picasa try this path on your computer and see what you have in it:
<your user name> -->My Documents -->My Pictures --> Picasa --> Screen Captures
I found 608 files (1.33Gb) of screen shots of me using Picasa from last year as well screen shots of my desktop!! I was like what the F***!
Edit:
Guess this is a feature but not straightforward to turn-off. Just scary to unintentionally find screen captures of your desktop.
