Another vote for Paintshop Pro here for serious editing.
I also REALLY like Microsoft's Picture It!
However, there is this little program that I just couldn't live without.
It's called ScreenPrint Gold.
Not only does it resize images effortlessly, it makes changing file types a snap (like Corky describes).
It does a few other things like cropping, flipping, rotating, brightness, contrast, etc.
To top it off, it re-enables your "Print Screen" button!
You can "Print screen" your entire screen, active window, or user-defined area. Then print it, or save it to file. It's great for printing those little "pop-up" message windows, "settings" windows, etc.
It also has a delay timer and an interval timer too, so that you can get screenshots of your favorite game.
Seriously, I have at least a half dozen image-editing programs, and a half dozen dedicated photo-editing programs on my computer, and I just keep coming back to SreenPrint to do resizing and converting.
Lots of Luck
R2