You can get ImageMagick for both Linux and Windows as a free download. ImageMagick is nice because it doesn't require root access to install.
Most webhosts have ImageMagick or GD2 installed as they are both pretty commonly used. Any gallery type software requires one of them to be installed. Just about all of the CAPTCHA systems out there use one or the other. Nowadays it's more common to find a host that doesn't have one of them installed than it is to find one that doesn't.
The problem with a bunch of images put together is they all have an actual filename. Someone that wants to automate their bot just has to take the time to see what filename corresponds to what image and they could crack it just by looking at the img src.
