I recently ran into a problem with uncle cheeses image gallery. I want to migrate the website of my hockey club to silverstripe, but since our hosting provider runs php via fast cgi we can't use more than 32MB of memory (even if you try to set a higher memory limit).
While searching for a work around I noticed, that imagemagick is installed on the server and i'm able to call the command line programms of this library (convert, etc.)
Therefore I have rewritten the GD class to use cli calls instead of built-in gd functions. This is a lot faster and the memory usage of called programms does not count to the limit of PHP.
I'll provide my class to you, but beware: it is not properly tested (just didn't have enough time), it won't check if you are able to use "exec". Also not every feature of the original GD class is implemented (e.g. paddedResize, which will call croppedResize instead).
This is for silverstripe 2.3, I haven't checked if the GD class of 2.4 is compatible.