I've been playing with this rather spiffing online embeddable image editor from Aviary, and it does exactly what I need it to, namely:
However, in the documentation, they indicate that this is only suitable for web-resolution images, while I need to allow users to edit hi-res images as they will eventually be sent to a printer.
Does anyone know of anything comparable to this Aviary editor that can work with hi-res images?
In answer to my own question, it turns out that if you pay for the Enterprise version of the software, then full resolution is supported.