I agree. I don’t see the problem in being able to upload whatever photo you want to your own pots; your Monzo account is private and not available to publicly view, so offence cannot be caused. The majority of people wouldn’t add inappropriate photos anyway.
I think it would make the whole Monzo experience better if you could personalise it, for example add a photo of the Eiffel Tower if saving up for a trip to Paris or adding photos of family members to save for their birthdays (or even a photo of your pooch!)
At the very least, more default options should be available.