For your information, I used the solution described by PGRated in this post.
With the 2.02.32 firmware, the WDTV does not use the thumbnail caching. So the trick no longer works. Shame, because it was possible to create a transparent image with Thumbgen and add information taken from IMDB for example, and everything was displayed instantly. It was a way to bypass the theme size limit on SMP.
WD, you do not help much themers improve the product. 
