Sure thing, maestro.

Just highlight the song in your iTunes browser and select File > Get Info (or Command-I).

Click the Artwork tab, click the current artwork to select it, click Delete, and then click Add and navigate to a graphic file on your Mac - or just drag one from the Finder into the box (we used a 250-by-250-pixel JPEG file, but GIF, TIFF, and PNG files also work).

If you have other songs from the same album, iTunes will only display your custom artwork if you change it for all of the album's songs.

To do that, click Album in the iTunes browser to group songs by album, Shift-click to select all of the album's songs and select File > Get Info (or Command-I).

Check the box labeled Artwork and drag in a graphic file from the Finder.

Then just click OK and you're done.