Calling into RMagick/ImageMagick from inside the request/response cycle is probably even worse than shelling out, because ImageMagick does grievous damage to your runtime.

Last I tried RMagick, it leaked significantly. Definitely not something I want to use in a long-lived process. I remember having to fork to use it, to work around the memory leak. If you don't need the fancier operations, there are lighter image manipulation gems out there that do just the basics, but without leaking. e.g. ImageScience http://seattlerb.rubyforge.org/ImageScience.html