Problem with overlapping images

Hello,

I’ve got some images with hover effects on the website. The use is really easy and it looks great one the page.

But sometimes when visiting the page the images overlap with each other. The whole installation seems to be destroyed. After reloading the page it looks fine again.

Is that normal or are the images too big (I think I used the size of 960x640px). How can I fix this problem?

Thank you for your answer and your help.

The page I need help with: [log in to see the link]