When you create an image with Photoshop or any other proper image editor such as Paint.NET you can save the file in its original state so to say - including all its different layers and used images. [See: https://en.wikipedia.org/wiki/Adobe_Photoshop#File_format]
So by allowing (and eventually encouraging) people to upload the original, editable .psd files instead just the .jpg / .png exports one would allow people to access these layers etc when modifying the image instead of just being able to modify the endresult.
This could be far more useful than it sounds in many cases.
Actually I've been thinking of creating such a site (there also the variations made with use of the provided image would be enlisted below the image) but it would be far more effective for Wikipedia to take over the job of truly open sourcing images.