All my DVD cover art for movies displayed correctly from Twonky onto my PS3 except for 2 particular videos.
It turned out the problem was that the folders containing each video had an ampersand character (&) in their names; e.g. 'Goodnight & Goodluck'. I simply changed the folder name to 'Goodnight and Goodluck' and the problem was solved.
I guess it's a bug in Twonky as it creates its own cache of the cover art and renames them for display.
btw, the actual video filename can contain an '&'; e.g. 'goodnight & goodluck.mpg'.