At last I found the solution, which I share even if mine was a bit particular as situation:
I should explain better what I did before. When I connected the first time my hard disk to the wdtv, and saw how it squeezed the images to make them fit in 2x3 vertical box, regardles of their real proportion, I created new images, from the old ones, but with a bigger canvas, to maintain the 2x3 proportion, using the freeware mogrify (ImageMagick).
Since they were still a bit squeezed because the wdtv interface expected a 16:9 monitor and mine was an old 4:3, I got better results with 2x3.6 images. But not for all the movies, in many cases I still got the old images.
At last, I have discovered that the wdtv was reading the previews saved inside some of the jpeg (Photoshop does that, for ex.) , preview that was copied from my old files to the new images. By cleaning these previews I got rid of the problem.
