In your iTunes library, highlight the songs to which you want to add the album artwork.
Choose "File," "Get Info" or press "Command-I" (Mac) or "Ctrl-I" (PC).
A warning message appears: "Are you sure you want to edit information for multiple items?" Click "Yes" to proceed. The "Multiple Song Info" Dialogue Box will appear.
Enable the Artwork Field by clicking in the empty box in the bottom-left corner of the window.
Drag the artwork image into the highlighted Artwork Field. If it's easier, cut and paste the image into the field.
Optionally, click the Check Box to the left of the Artwork Field if you want iTunes to automatically change/add an image for the entire album. Remember that when you add a check mark to a field, iTunes will change the field in all selected songs, so be sure that only the Artwork check box is checked.
Click "OK" to finalize the change. Download time varies depending on how many songs the art work is being added to, but the average 12 song album should take no more than one minute, depending on the computer's speed.