Thanks for raising this! (And welcome to the Prismic community forum!) You're not the first user to ask this question. It has come up before, and you're right — image URLs are definitely a ranking factor for search engines. There is currently no way to remove the hash. I've submitted a ticket to our dev team to see if there's anything we can do about this, and I'll let you know what they say.
In the meantime, Prismic images otherwise have very strong search engine optimizations. They are served with automatically optimized image format; we have built-in responsive sizing; we have built-in alt tag handling; and we serve images from Imgix's CDN, so they are very fast.
For the filename, there are some advanced workarounds possible, but I wouldn't recommend them, as they would require a lot of work for small gains. But, if you'd like to know more, let me know.