Well actually, in this case both versions are somewhat concave (I can see both now). It differs a bit with each illustration. But generally, if it's a simple image of say a ball poking out, the concave version will just have the 'ball' become a hole into the image instead.

apologies, i'm explaining poorly! the overlaid colours is what i meant by holographic-esque, and the iris of the eye looks convex. so you're viewing it the way i assume it's supposed to be viewed

the other way of seeing it, the iris of the eye looks concave & the colours of the whole image look more or less like the 2D version of it, they don't merge/overlap