Do you use textures or overlays in your photo editing? If so, you might be interested to know that Amsterdam’s Rijksmuseum allows you to download and edit images any of thousands of objects (paintings, sculptures, drawings, furniture, clothing …).
And once you’ve downloaded the image, you can select parts of it to use in your editing.
If you want a real challenge, how about showing a framed version of a famous work hanging on the wall of a room you’ve photographed.
Or do something like this (recognize the background?).