Problem with movie covers in library mode under Frodo

Hi,
I installed last release XBMC Frodo to my PC with windows and I have following problem :
In all directories I have movie.tbn file with cover for library mode, but when library is readed, some of movies are without cover, I can see only screenshot from the movie. This problem is new for me, because under Eden and also Dharma was all ok. Is there any change which is reason for this ?

Thanks

Alda