Image Size

Hello,
Yesterday I created a forum with a purpose. Sometimes people have to post screenshots there which are bigger then the forum is wide, the problem is that the forum doesn't automatically resize the pictures so you see them completely on the forum.
I have searched in the ACP but I couldn't find a way to let the forum resize the pictures. Now I have 2 questions:
- What is the maximum screenshot size (in pixel x pixel)?
- Is there a way to let the forum resize the screenshots?

I'd really appreciate it if somebody could answer at least one of these questions,

Sincerely,
Karel