Here's a little tool thats been around a long time and quite popular in situations where images need to be created using scripts (captchas, etc). It's also excellent as a tool for mappers.
It's command line only, versions for windows, linux and mac and comes with a set of utilities.
The most common utility is called convert, it can be used to convert image types, resize, crop, create images and a lot more.
Creating images using imagemagick
For a full list of all the capabilities see Imagemagick use
I use Imagemagick tools quite often, I always strip out exif and ipct information from all images (even my camera writes this information to images) and this information can sometimes bloat images. I can strip this out using a single like
mogrify -strip *.jpg
I use the utility to resize all my images (using a script) so I always have power of 2 images.
Once you start reading into the capabilities of this tool you'll find a great many ways to use it.