I was looking for something like that (hosted) yesterday and I stumbled upon cloudinary[1].

The do the resizing part for you as well as the hosting. But the best thing they offer is face recognition. It's incredible! I haven't been in live with a service since guthub.

Their free tier is probably enough for smaller projects (but without backups in your S3 bucket)

Thumbnails with focus on the face looks fantastic!

[1] http://cloudinary.com/invites/lpov9zyyucivvxsnalc5/bwrrdazeh... (affiliate link. Without: http://cloudinary.com)