The easy way...
  • Locate the artwork you want to insert for a particular song.
  • Copy it to your clipboard.
  • Select the song in iTunes.
  • Hit command-I.
  • Click on the image box in the upper left hand corner. (It will become highlighted in blue.)
  • Hit command-V.
  • Your image will be inserted.
And if there's another iTunes location into which artist pics can be inserted, neither I nor Google can locate it.

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire