I like this idea. 
It might be worth updating one of the welcome bot’s text strings to let people know that, as they’re only testing the feature, their images won’t be stored long-term unless they re-upload them.
It could be spun in an encouraging manner; “feel free to pick any image - this is just a test run!” or similar would convey the information, while focusing on the positive - in this case the user not needing to worry about finding the best image, and instead just using whatever file is convenient. This is one of the things I really like about the Discourse team - their messaging always feels friendly and welcoming.