Brian

It depends on the server. If you want to try it out real quick, switch the resizing program option to IM, then try these for your 'Resize Program Path' option:
/usr/bin/
/usr/local/bin

Those are just the most common locations. If they don't work then I would contact your host and ask them if they have IM installed, what the path is, and if they don't if they could install it for you. Most good hosting companies will not have a problem with installing IM.