Album Artwork

From within iTunes, select the song and select "Get Info" and pull up the artwork tab. You can then click on the artwork and select delete.

You can also group-select multiple songs and delete the artwork via Get Info by putting a checkmark in the little artwork window, but don't put anything in the window and select OK to remove the artwork to all the songs.

Why do you want to remove artwork? If its to save space, you'll be disappointed with the minimal savings that will get you.