This is pretty cool. This week a small startup launched called Transloadit. Self titled as "Fantastic file uploading for your web application," it seems like this might actually be pretty fantastic. Once you sign up for this (pay) web service you can point your upload forms to it and it will receive, resize and store your images to S3 for you. They even provide a nifty little jQuery plugin to make nice upload forms.
Image processing is always the hardest part of an app and this does all the hard work for you. Take a look and see if it might help you!
Launch Blog Post: http://kevin.vanzonneveld.net/techblog/article/announcing_transloadit/
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