If you're considering the DIY server-side image resize route, here's a good list of options:

  https://github.com/adamdbradley/foresight.js/wiki/Server-Resizing-Images

... and a comparison of benchmarks – ImageMagick vs. competitors like the very fast, low memory VIPS:

  http://www.vips.ecs.soton.ac.uk/index.php?title=Speed_and_Memory_Use