Copyright, buying permissions and all of that stuff:
1) At some point SOME kind of line has to be drawn. If I draw the outline of a horse and do an half-way decent job it is going to look like a horse. But then, how many tens of thousands of other images of the outline of horses are there in the world? And should I worry about copyright? Cripes, do we have to go all the way back to the Caves of Lascaux in France to say who was the original author of the "horse image"??? (pardon my exaggeration, please)
To me, there has to be two elements involved: 1) the original, whatever it was, has to be sufficiently novel (as in patents) or distinguishable or the like so that you look at it and say "Oh yea, thus-and-so obviously painted that." Kind of like the Mona Lisa. You have to get pretty far away in characteristics to consider your work non-derivative.
2) and then there is the point of it being derivative at all. My random-forces-existing-in-the-galaxy-affecting-all-things (uh, this is supposed to be a non-religious version of the word "God"), there are just simply not that many variations on the "pinata" theme - or even one that shows an "x-ray" of what stuff is inside of it. So, if an image has become commonplace, then there can't really BE any question of copyright because, in effect, there is no undeniable source.
Do you have the SLIGHTEST idea how utterly determined the Band-Aid(tm) people are about defending their trademark at this late a point? Their lawyers are absolutely ruthless. You'd better not copy THAT name!
These, of course, are two extremes of one metric, that of imitation & whether or not one must pay homage to the original artist, or obtain their permission, or pay them a license (or whatever) OR well, gosh, it is just like everyone else's so you just can't complain.
3) The true problem is where, for a given image, do you put your finger on the line. Unfortunately, this is totally a matter of opinion (actually, it isn't entirely so: programs like d'peg can compare two JPGs, come up with a similarity index, and ask you if the two shots are actually awfully darned close to each other and do you want to drop the duplicate).
So: who is to be the judge who puts their fat finger onto the line of similarity and judge if something is too close to an established artist's work and therefore special consideration need be made?