I recall you noticed this before ... this happens when changing the background image in a forum and then it also shows in other forums?

Yes- it shows up temporarily. 

Also it seemed to just effect certain forums so there may be some clue or commonality to that - if it's true.

Does anyone know WHAT that graphic is? This is a tiled (repeated) image. I was hoping knowing what the image was would give a clue as to which forum might have installed it.

It's  a common image, comes up when googling for background images. I thought when you switched backgrounds you saw the image you were using in other forums?

Wait!!!!!

It might be when I used mobile to switch backgrounds! 
IDK but I just came across the image I used way back when Paul first noticed this and it was stored on my phone. 
https://forums.delphiforums.com/hosts/messages?msg=28949.3

So I used the image from the previous message on one of my (many) owned forums, uploading it with mobile. Backgrounds on about half my own forums changed plus many I do not own.

and many many more (sorry folks)

I'll try from desktop in the morning.

:D

neato ...

Right! 
Anyway I'll try from desktop tonight when its a little less annoying to the multitudes