1. Right click on the song. 2. Go to "Get info." 3. Find an empty box which says "Album Art" 4. Drag whichever picture you like.
You can also select multiple songs in the list, right-click, get-info, then drag the album art in the empty box. This will set the picture in all the selected songs.
thanks guys. took me a day and a half. but finally done. i only have about 250+ songs. wonder how those people with 80Gb of songs do this?