Arguably, it has nothing to do with racism andveverything to do with making fun of a political opponent for being an idiot. After all, liberals referred to President Bush as a chimp on a regular basis.

For example, we have this from the Huffington Post:
  • Prior to reading this new study I was under the mistaken impression that Bush was a parrot, because of his apparent inability to say anything about Iraq except 'progress is being made', which he repeats mechanically every time the topic is raised. But now the scales have fallen from my eyes: Bush is a chimp. It seems so obvious now, watching him speak.

Were all those references racist?

Assuming that, yes, "chimp" = "racist", now google the terms "Bush chimp" and then "Obama chimp" in image search. You get approximately 77,000 more images of the former than the latter. That an approximate 38% difference.

Most likely, in each case, the posters of same were doing so in an attempt to insult a political foe.

And, if we follow your argument that depicting a president as a chimp is racist, then it would seem that there are at least 38% more racists on the left than the right. That's a pretty big difference.