I'm going to be very honest here, and I really apologize if I come off as a downer.
While that's an interesting idea, I don't think any world creators are going to pick up on it for several reasons - the number one reason being the worlds they share are generally the result of months (sometimes more) of their own work, if anything, if they had custom images on their billboards, I'd imagine they'd want to design them themselves (like Neuroticbiotic created custom images for the stadium/theater/etc in Eltham's Drift and Glendalough).
Some creators avoid CC, and many avoid stuff like English in their games - players are the same way, particularly when it comes to custom worlds more than anything else.
I noticed a general CC-Phobia from a lot of players on the officials, even though this kind of "CC" is actually just a custom resource and technically doesn't even require them to download a separate object/package and can be included directly in the world file. :\
Another drawback is that the world file would have to be updated to update the image - or if it's included externally, CC package re-downloaded, so an advertisement would literally be almost permanent. This makes it a bit less versatile. Worlds for the most part are not created with the same frequency as sims, lots, or CAS parts.
There's often a certain aesthetic or theme creators are trying to capture with their world, and details like this are a part of that aesthetic. As for the popular builders on the officials, I sadly don't think that Johnnywr (created Great Bear), RFLong, or honestly most of them would be on board with this idea. These creators are generally EA-match. You might have more luck approaching some different people. And it's definitely something interesting you can add to your own worlds, of course.