Why image will disappear some minutes later

Enclose is a menu movie that I made for my new website.
The effect for the images is like that, the 3 thumbnail pictures are loaded from an external folder, the txt file is very simple. When users click one of the 3 thumbnail, then the larger image will be shown.
The problem is that after the larger image shown, some minutes pass, the selected pic would disappear. Why? Can someone help me?
Sorry I am not good at English.