Abiathar, there are two important reasons why humans cannot depict a being more beautiful than themselves:
1. Human imagination is actually limited - limited by human experience. We simply cannot imagine what we have never experienced. Every image that you can imagine is constructed of elements that you have already seen. Human imagination basically takes elements from our memory and re-arranges them and morphs them to produce a new arrangement of those elements.
2. Beauty is a very species-subjective "sense". Seeing others of the same species as being the most beautiful of beings is likely a natural consequence of, or contributor to, our evolution as a social species. Seeing others of the same species as beautiful helps to foster attraction and cooperation between members of the same species. We don't see the human form/face as the most beautiful because of it truly being objectively the most beautiful, being divinely created. No. The brains of social creatures evolve to perceive members of its own species as being the most beautiful. I'm willing to bet that chimpanzees see other chimpanzees as being more beautiful than humans.
So you're right, we cannot conceive of a being more beautiful than ourselves. But that's only because of the highly subjective, species-specific nature of the quality of perception we call beauty. The moment a form begins to deviate too much from the human form we no longer perceive it as beautiful. Beauty is a perception based on the wiring of the brain. It is not an objective reality that exists outside of human perception. So it is not that a more beautiful being cannot exist. It's just that our brains wired with a bias to perceive others of the same species as most beautiful and the same is probably true for many other species (they too are wired to see members of their own respective species as most beautiful)
So you cannot objectively compare the beauty of different species because each species perceives beauty with an inherent, inescapable bias based on criteria specific to its own species. You can only compare beauty within a species. So the idea of humans not being able to portray a being more beautiful than themselves is a non-starter. It's actually not very different than asking: "Why can't we conceptualize a physical being that is more human-like in appearance than humans?" Or "Why can't we imagine a color that is redder than red; greener than green; bluer than blue, etc?" LOL.