I know there are plenty of examples of apes making paintings and drawings, but has there ever been a recorded case of one representing something in their art?
Yes. The objective clarity of what constitutes representation is not certain, but here: https://www.youtube.com/watch?v=nSHQyLsPm4Q you can see something relatively convincing. The elephant really slays this, but your question is about apes; I don't know how species-specific you intend to be.
I find that non-biologist laypersons assume that finding humanoid traits necessitates searching in human-like animals. Often not so! In fact, I wouldn't be surprised if someone came up with an octopus capable of a transformative masterpiece.
Part of the problem is ascertaining whether something is representative, outside humans primate lack the fine motor control needed to make realistic art. And normally you cannot ask them what something is. individual chimps and gorilla have painted or drawn and clearly take pleasure in the activity without any reward.
However there is one way in which the ape could tell us what their art is, some chimps and gorilla know sign language.. Both Koko and Micheal the gorilla sometimes named their paintings, this is by no means conclusive but the combination of colors and titles is suggestive of a the art being representational. however no exhaustive study of their art has been done to determine if this is cherry picking the data.
Likewise chimps who both know sign language and paint like Washoe, will title their art and when later asked identify them as the same title. More notably some claim their art patterns to their art, when asked to paint a dog for instance Washoe produces similar shapes each time. However again these are interpretations by keepers not independent studies, which it should be noted would be very difficult given the language skill of the animals. But like all behavior science, conclusiveness is nigh impossible to achieve but they are certainly suggestive even if they must be taken with a huge grain of salt.
It is also worth noting apes there is a notable difference between self motivated and reward motivated "artists" those prompted to paint for a reward show significantly less care, planning, or evaluation but just make something quickly then demand a reward.