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.
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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.
Stock photos is a feature that exposes a tension in our product - what's good for sellers isn't always good for buyers.
That said, you also make some good points why stock photos are bad for both.