The issue is the name of your photos is incompatible with a background style tag.
http://www.travelstories.gr/photo/da..._(armonio).jpg
Basically a background url styletag goes like this
backgroun-url(
http://www.travelstories.gr/photo/da..._(armonio).jpg)
The parentheses in your filename break the background tag. Only way around that is to rename your files or do not use the overlays.