Well it looked like you were asking for a picture to prove that she wasn't wearing a belt. Which I know sounds stupid and isn't very lewd at all, but it felt similar to someone asking for something else.
On a serious note there is the Cobo Center and the Renn Center.
If you're going to draw dicks on walls then it would be fitting on the city hall.
There's also a big metal statue of a clenched fist hanging like a battering ram. You could use that as a prop for lewd fistings if you get the perspective right.