OS 10.5 - Adding artwork to ID3 tag instead of separate database in post-iTunes 7

Because I want the launcher Quicksilver to display my artwork when it changes track, I want my artwork to be in the ID3 tags of mp3s and not in the separate database in iTunes 8. On the Quicksilver forums I came across the following script:

(*
"Embed Artwork" for iTunes 7 or better
written by Brian Webster

- v1.1 sept 25 2006
- added routine to check for a selection, account for de-selection of
tracks (D.A.)

- v1.0 sept 25 2006
- initial release
*)

tell application "iTunes"
if selection is not {} then
set sel to selection
with timeout of 300000 seconds
repeat with aTrack in sel
repeat with anArtwork in artworks of aTrack
if downloaded of anArtwork is true then
set theData to data of anArtwork
set data of anArtwork to theData
end if
end repeat
end repeat
end timeout
else
display dialog "Select some tracks first..." buttons {"Cancel"}
default button 1 with icon 2 giving up after 15
end if
end tell

I pasted it into Script editor and ran it but it said "Expected end of line, etc, but found identifier" and had the word "button highlighted"

What is wrong with this script? Or how should I otherwise convert all my artwork?