From a quick glance at Thumbor, it seems it also offers object detection (like faces), which is a nice add-on. Nice post, I didn't knew about this project, even though I'm working on the same space!
We'd love to see on the fly retina display processing added to it too ;)
First, I came across timthumb[1], but it was a little too heavy for my liking. Instead I whipped up a thumbnail-on-demand script with caching options in about 3 hours and 80 lines of code.
Now the best part -- I don't have to use a work-queue on the server side to offload thumbnailing. The browser will do that on first request, and results are cache from then on.
I think Cloudfront has a way to pass variables to your server so theoretically you can use any image generation package right?