I've been using the *.jpg-format because it's the only format which is available in both programs.
If Hardcopy does not support PNG I wouldn't use that anyway PNG is WAY better
for screenshots than JPG.
Thankfully you discovered a really great program now, just play around a bit with Screenshot Captor and discover all the nice possibilities!
@mouser: google for "free snagit alternative" - it does NOT yet show your program in the first 10 search results! Since it's so great and free we need to change that, I suggest you add some words in the description on your page.
Let me start: Screenshot Captor is a great and free alternative to SnagIt, I really like it a lot!