I was just trying to find Fiona some PC-specific help, and knew that Harry had written something up somewhere. Didn't want to reinvent the wheel...
I found what I was looking for: Harry's excellent guide to cropping & resizing image files on a PC
. I thought I had posted how to do this with the built in image editor app on a Mac ("Preview"), but it must have been on a different forum. Maybe I should write one up and it could be stored with the PC version.
However, if anyone is really interested in working with their image files I recommend learning something a bit stronger than Paint though. First of all, shoot your images in RAW, then you can fix exposure problems much more effectively (files are bigger, but memory cards are dirt cheap now). Then use something like Photoshop (even the Elements version is enough, there are Mac & PC versions) and you can bring out shadow detail, darken and "moodify" overexposed clouds etc. Photoshop also has a superb function called "Save for Web", which allows you to reduce the dimensions of the image as well as the colour-set. The file size reductions can be extraordinary, and you get to preview the effects before you save.