Simply saying... my online exams wouldn't also work for the visually-impaired.

Says who? Visually-impaired doesn't imply totally blind. People can have all kinds of restricted vision disorders. Some may be unable to take your exam - but some may be able. But using a distorted image as firewall can be a hurdle too high to take for some people able to take your exam.

OTOH, if it's just a site that's about online exams which can't be taken by visually impaired, why the distorted image in the first place? Afraid the bots might take the exams?