Don't know if anything like that is available, but there are a couple of programs that, when used together, could simplify things.
Fraps does more than benchmarking your fps: "Screen Capture Software - Take a screenshot with the press of a key! There's no need to paste into a paint program every time you want to capture the screen. Your screen captures are also automatically named and timestamped."
Irfanview can do batch conversions from one format to another.
They are both free (Fraps 1.9d is free, 2.xx is feature-restricted shareware).
EDIT>> The above recommendation is of course, for Windows. If using Linux, KDE has a built-in utility to do just what you are asking:
ksnapshot.