Almost all audio players have the ability to download “album art” for the tracks stored in your computer or imported into the media library of your browser. The fact that some of your files do not have an album art means that the ID3 info is missing or the tracks are in an unrecognizable format regarding the metadata of the file. The easiest way to fix this is to select all the tracks without album art and select Auto-Tag and wait for the tagging process to finish. Then the player will be able to download the album art when the player is in idle mode in order to conserve traffic or speed.