I was recently looking for services that could handle resizing and cropping. Personally I prefer the ones that act like a proxy, so I can store the images in my own S3 bucket. Here are the services I found:

http://www.imgix.com/ http://cloudinary.com/ http://www.cdnconnect.com/ http://cloudresize.com/ http://cloudimage.io/ http://www.blitline.com/ https://transloadit.com/ https://uploadcare.com/ http://www.resrc.it/