Unit testing a complex renderer class (Best Practices)

I'm trying to test a class which purpose is taking a specification of what to do with a BufferedImage array and proccess it.

I'm concerned about how to aproach this task, it makes no sense to me just duplicate the function code on the test in order to generate a "expected bufferedImage array" and check if the generated BufferedImages are equals to the returned by the class methods.

I've read some slightly similar questions here, but the answers was "charge expected images from file and check with returned ones by class methods" but that sounds real hard to maintain if the proccess changes, new methods are added to the renderer class, it's hard or imposible pregenerate a resulting image or simply the class is refactorized and functions are splitted.

My actual question might be: Is there a little more elegant, "better to maintain" way to do that. I haven't so much experience with unit testing so i'm not sure about how to do this.