Dotted frames around pictures in FF

On http://mohsart.se/art I have some drawings and stuff. I am pretty content with the layout, although I will make some minor changes like background colors and different thumbnails for drawings and folders with drawings in them etc.
However, there is one thing that disturbs me a lot: If I click on a thumbnail to see a drawing and then go back, the thumbnail is left with a dotted line around it.
This does not happen with IE, which is even more disturbing since I prefer FF.

If someone has a tip on how to get rid of this, please let me know!