I don't play TF2 very often, but I know there's a sign you can put an image on. And, every image i've seen of it is either censored or disgusting. Even if it was text only on a Yeeps sign, there's a lot of harm you could do with chat filter bypasses, and a few chances to ruin a child mind if you can dodge bans.
#Why I say no to signs.
storm hinge
prisma dune
Add a report button directly on the sign