Are they innately tagged through id3 or is it a file?
Get the album cover (at the right resolution) for the songs in question and make a file called (songname).jpg and add it to the same folder as the song.
The X3 is able to support album cover either through direct tagging through id3 or if a file is also in the same folder with the same name as the song but with the jpg file format.
I think they have also added support for having a file named 'cover.jpg' in the folder and using that.
Also make sure that X3 settings have Cover as the main display option in the settings.