Exactly - Yoshi already responded with a similar answer a while ago about using custom images for FC logos, and they stated it would be far too difficult and time consuming to have to moderate every piece of art that is submitted by players for use as their FC logo to ensure it meets the guidelines of the ToS and EULA (not being offensive or infringing on copyright).

Hence why we're locked to using combinations of default premade artwork. It would be the same thing with housing pictures and statues.