When you edit the item on the web page, you can replace the picture (both from file, URL, Marketplace, and from texture UUID).
It's a bit hard to respond to the issue due to missing details, which I'd need to reproduce it in case this is actually a bug. It might also have been that an item was added based on a Marketplace search, in which case the search result might be inaccurate (e.g., if the respective item isn't listed on the Marketplace).