Safari heavily distorted image

I'm experiencing weird image distortion with a single image on a single website in Safari. I haven't been able to reproduce it with other browsers or other images, but it doesn't happen very often, so I'm not sure whether it's only Safari.

The website is www.laurensdressuur.be .

I had my laptop repaired last month for the typical MacBook Pro GPU problems, and I hoped that would solve this as well. Clearly it didn't. I'm just wondering what causes this. Is it software or hardware?