The space of easily visually distinguishable images has a certain size. Let's assume there's a deterministic, pseudorandom mapping from domains to images. For a given domain, how many plausible impostor domains are there? What's the chance that there's at least one impostor domain that happens to get the same image?

If you have 1000 distinct images, but a given domain has 5 letters that could each be replaced with any of 3 visually identical Unicode characters, then, well, the chances are very high that there exists a plausible impostor domain with the same image. I don't think this is a very workable approach.