Image cropping before uploading

I'm creating a website where the user can upload several images to his album, the thing is that I want to allow him to crop the image, specially because he might upload a very vertical image and we want them to be at the same size and aspect ratio.

Gravatar for instance does that way:

I want to allow the user to select an image, crop and then continue selecting images, like this:

And when he is done he will press upload and that is it.

How can I accomplish something like this?