If your web host doesn't allow you to use Imagemagick, here's a substitute for TYPO3

Using Host1free, I found that ImageMagick and the exec function are disabled. That's no good. Luckily, there's an extension called jb_gd_resize that aims to allow image resizing with GDlib2. Be sure to check out some problems with jb_gd_resize and possible workarounds here.