Can't add artwork to itunes movies

Hi,

I am using Video Converter Ultimate to load my dvd collection in iTunes movies. The converter output is .m4v file. When I add artwork to the "Artwork" tab of the Get Info box It will add but then I get a spinning colored ball and I must force quit iTunes. When I reopen the move in iTunes the movie will not play. The move is there but when playing the movie it is black (no video will play). Any suggestions?

Thank you

VPH