If you have control on the server to install programs via the ssh then you can install imagemagick based on these instructions. Some vpns actually have imagemagick as a program you can install from your web control panel
http://www.photopost.com/installphp.html
Yes 0 is unlimited so if you test and see what sizes your gd can handle and can not you can limit users to upload ads to a certain size so they dont get the white page