Answer:
1. Find the artwork online, and copy it to your desktop.
2. Open iTunes and play the song/album/band.
3. Click the button on the bottom left of the iTunes window that has a rectangle with a triangle inside it. After you click it, the triangle should be pointed down.
4. A box will open on the bottom left of the iTunes window that says either "Selected Item" or "Now Playing". If it says "Now Playing", skip to number 6.
5. If it says "Selected Item", click the circle with a triangle inside it (to the left of where it says "Selected Item") until it says "Now Playing".
6. Once you get to the "Now Playing" box, you should be able to click and drag the album artwork from your desktop into that box (It says "Drag Album Artwork Here" and is surrounded by a dotted line.)
I hope I've helped!