A great alternative to IM and GD is Imager - it's much, much nicer for resizing than GD (can't speak for IM since I've never actually gotten the library to install).

Two problems with it - one, it doesn't seem to be maintained anymore (anyone have any info on this?);

and two, it seems (at least to me, maybe I am missing something?) that you can't scale the axes independently, in other words, it will always constraing the proportions for your thumbnails (I've been using it for over a year before I came across this limitation, but when I did, I couldn't get around it in Imager and had to use something else).