This back part of the body camouflaged itself quite well, still we can see it's an arse.
Using the letters 'A', 'S', and 'S' or 'A', 'R', 'S' and 'E' in combination with '8' and 'R' leads towards
ASS8R or ARSE8R
Translating those with the help of the alphabet turns into '1 19 19 8 18' or '1 18 19 5 8 18'.
Adding those numbers up results in 65 or 51. Deal with it.
Anyway, those numbers give me nothing.
On the other hand, "8R" i.e. "EIGHT AR" sounds like "HATER" or "EATER".
Now we could go for ASSHOLE and HATER. History has it, that some riddlers here are strongly opposed by the mods. Therefore the rebus could be some kind of goodbye message for the mods. I.e. describing them as the assholes and haters they like to be (from a riddler point of view).
However, by looking closely at colours, we can see that the characters '8' and 'R' are all black. Another thing that's black, is the moustache
(whereass the hat and the body part were coloured in many hues).
If you tried to find a letter that looks like the moustache, you'd probably find the letter 'W'.
Combining it with the other letters gives you 'W', '8' and 'R'.
In other words:
W8R = WAITER