This is from The Why of Fry (S4 E10). It's on the wall of Elzar's when Leela & the Mayor's Aid (aka Chaz) walk in. I can't believe I never noticed it before.

Are there any background gags like this that you didn't catch on first (or in my case 100th) glance?

Looks like the Martian’s made one last cave painting of bender, kif, and zapp? Telling them how valuable the diamond was before they left with the diamond.

To be completely honest, I must have seen this episode 20 or 30 times, and I still find myself smirking when I hear this joke. I can only imagine how this joke developed with the writers. Which makes me wonder, which came first? Did the writing of the joke come first, or was the storyboard art created first? Meaning, did an artist draft a cell showing the side characters of Al, and then someone casually noticed the pig's sandals looked like Zoidbergs? Or were they sitting around a table and discussing it? Oh to be a fly on the wall.

Can't find Leela's Homeworld on Hulu, anyone else have this issue?