Ipod artwork problem?

iTunes has an option to select "Convert higher bit rate songs to 128 kbps AAC" in to reduce the space for the iPhone, iPad & iPod. Select that and then group select those songs by selecting the first song, then hold the shift key while selecting the last song. While those songs are selected, right click and select "Clear downloaded artwork". Of course you will not have any art work for those files at this stage, but you can attempt a new sync to see if the problem is resolved. Alternatively, you can download a free system optimization software program called Onyx that can located corrupt files & preferences.