Serving a giant blinking red gif to unsuspecting internet users is a bad idea. https://en.wikipedia.org/wiki/Photosensitive_epilepsy

There was a better idea posted in comments - serve a picture with a very short explanation and an email to contact.

There is no reason it needs to blink quickly.

It doesn't have to be quick for bad effects. See https://discussions.apple.com/thread/7908738 for example. I can't find the link now, but there was also an article about one of the older designers (IBM?) making sure the terminal cursor blinks in 1:2 ratio to reduce the problem.

Just don't give random unsuspecting people blinking images as a rule.

The first link says "several flashes per second". I was thinking more like a blink every few seconds, which seems to be annoying but not dangerous.

But he's suggesting that they only be served to 10^-6 people - that's one 1,000,000th of a person - I suspect it will have little effect