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NASA Link: Curiosity Navcam Sol 2414, Returned May 22, 2019.
The rounded objects upper left sure look like human skulls, eye holes seem visible in the one top left.
Frothy @ China Forum says, "I've zoomed in even further, check out the background."
I take that to mean you have discerned the object is part of a larger structure which discounting pixelation is revealed in the enlargement..
Opportunity: Navigation Camera: Sol 4689.
You could be right .. the background to the Metal Bracket & Flange from Opportunity Sol 4689 above when processed with computer image enhancement software, resolves into a mass of machinery which to my eye bears certain resemblance to the pumping systems one might encounter in an oil refinery.
Opportunity: Front Hazcam: Sol 3063.
Terry Stokely @ MAA who posted the Mars Snake from Op Sol 3063 onto YouTube says he can see "machinery" as well .. I found it more difficult to discern particularly in the enlargement, though sometimes I get his drift when viewing the thumbnail insert .. we all know you "get nuthin' for nuthin" you have to put in effort for this stuff, thanx for the reply .