How to store iTunes cover art as mp3 id tag?

This is the solution I found until now. Since it's very complicated and time consuming I'm still looking for something more convenient.

1. In iTunes, mark the song(s) you don't have a cover art so far and choose "Load Cover Art" from the pop-up Menu. If iTunes has some halfway correct information about the song, it will find matching cover art for almost every song I tried.
2. Now choose "Information" from the pop-up menu. In the dialog, go to the "Cover" tab and click on the cover art. Now you can copy it to the clipboard with "Copy" from the pop-up menu. Then close this dialog.
3. Now select all songs of the same album and again choose "Information" from the pop-up menu. On the "Info" tab, right click on the "Cover" square and choose "Paste". Now you can close the dialog.

This stores the image in the MP3 as an ID3 tag.

Mp3tag is a nice free tools that allows you to edit or download meta data including cover art for your MP3 files.

Mp3tag supports online database lookups from, e.g., Amazon, discogs, or freedb, allowing you to automatically gather proper tags and cover art for your music library.