Unfortunately, it's not yet possible in Nuxeo through contributions.

The easiest way to do that is to add one listener (synchronous or asynchronous, it depends on your needs) where you will do the image resizing based on the original image and override the existing Medium and Thumbnail views.

You can also generate new views in this listener for your custom sizes.

You can have a look at AbstractPictureAdapter#createPictureimpl to see how the resizing is done.