I’ve been wondering which image toolkit is likely to result in lower resource usage (especially coming up against the PHP memory limit) for applications like Gallery and Drupal. People used to always say the resize quality was better with ImageMagick versus GD, but I think GD2 closed much of that gap. But what about resources? The best answer I could find was in a thread on metafilter.com where one respondent says

if possible, do yourself a favor and switch to ImageMagick as soon as possible. GD2 runs within the php child, so resizing any image over 640×480 will pretty much take down the process you’re using. On our server, we’d have children allocated 64megs of memory die trying to resize a 1280×1024 image.

While I’m not an expert on server tuning or linux, my understanding is that in your typical usage scenario, you’d run IM as a niced shell process, meaning that 1) the php child won’t crash because it just launches the exec and continues, and 2) the process shouldn’t significantly slow down your server, because it’ll only use ‘free’ resources. This CAN mean that your user may experience a delay between uploading a file and being able to see a thumbnail…

That’s the best I can find for now. I should test it, of course, but for now I’ll take that advice and default to IM until I run into trouble.