You should pre-populate the images with stock photos found on google images. Removes one more barrier if I'm going to use stock photos anyway. (Obviously clearly mark it as "stock photo", still make it easy to upload my own). This is really cool by the way.
I think the text inviting them to upload a photo is fine. It's simple and obvious (though as I mentioned elsewhere they might want to offer an affordance for drag and drop). A stock photo might introduce more problems than it solves. The user might not realize that they can easily change it, it might be slightly irritating if the image is 'wrong', and they'd have to find a set of images that are free/cheap and safe f…
That said, you also make some good points why stock photos are bad for both.