Well what you want to do is find a high quality image of the album art (I usually go into the get info section of a song, go into the album artwork, and drag it out into the desk top).
When you open iMovie, go to the lower right hand menu where there is the picture/music symbol. Click the music icon. Drag out your song into the creation plane. Then click onto the picture icon to retrieve your picture (you usually have to place the picture in the iPhoto folder, works kinda like bridge in photoshop) and drag your picture over the song.
You're not done there. You'll notice that you will only get about 4 seconds of the song clip. Scroll over the segment to see some small icons. One of them will have one for the picture segment. Insert the time for the picture as the song's length.
Next, you may notice the really annoying Ken's Flare (or whatever it's called). That can be fixed by editing the camera functions and turning off the Ken's Flare. It should be the third (and final) option in a drop menu.
Test play your song some to make sure it is looking okay, and then you can export it into a video format (.m4v is highest quality).
If you have any problems contact me. My explanation might be a bit vague since I don't have iMovie open currently.