Images (not loading clearly every time)

Afternoon all,

All my images seem to load ok, except for the ones associated with my prices – I thought it might be a problem with these specific images but I can’t see anything obvious.

My site is: https://www.learnandspeakenglish.co.uk

Basically, when you click on any of the 6 links in my prices section, it brings up a page showing 3 images, and they’re always blurry. If you click on one of them, it brings up a page with the same image, which isn’t blurry. And if you navigate back to the original picture, it’s no longer blurry. Basically, it’s the initial picture that’s always blurry.

Any ideas?