This image of a cross in a circle on the surface of Mars is real. It's from the Opportunity rover, Sol 3720. It's from the Microscopic Imager camera, so it's quite small.
The most obvious clue is that it look like it's in soft sand. You can see cracks in the crumbling sand to the left of the cross, and there's a curve right next to it, that looks like something was pressed into the sand. So the obvious candidate for the source of the cross is the rover itself.
And it turns out there is a very simple explanation. The x-ray spectrometer, seen here as it currently is on Mars:
Here's a close up of a test version of the instrument.
This is pressed against the rocks it is sampling to get the most accurate. Notice the screw heads are recessed. When this was pressed into the sand the cross came from the screw head, the circle came from the recess, and the curved line is from the inside curve of the instrument.
The screws are a variety of distances from the inner curve:
Here it is in action, pressed up against a rock: