Previews of photos disappearing

[My-rin]

Hi! Here's the problem with using Pagemap ImageWall on the website: I've made square samples of each photo (.jpeg) and placed them in folder Cache. For several days everything's alright on the website, but after three-six days some previews just disappear, leaving the black squares on their places. I do the same action again and again: go to the WinSCP and load my previews, but they disappear again and again. Did somebody have the same problem? What could possibly be wrong in code?
Thanks for the answer!
P.s. The site is mine, but I'm not a programmer unfortunately (just understand a little in HTML). So I'll be grateful for as simple explanation as possible