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.
That's useful as a tool for a manual search, but you can't trust it by itself, since sites can tag any image as CC. In fact, you can find very well known -and definitively not CC licensed- photos on that search, like World Press Photo winners or Leibovitz's.
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 for commercial use.