Make sure your computer is connected to the Internet. On a Mac running OSX, click on the iTunes icon in the applications folder. If you are running Windows, click on "Start," "All Programs," and then "iTunes."
Click on the "Music" tab in the left column. A list of all the MP3s will appear.
Select the MP3 you want to fix by clicking it once to highlight it, then selecting "Get Album Art" in the "Advanced" menu on the top of the screen, or by holding down the control key and clicking on the song and selecting "Get Album Art." iTunes will scour the online database and find the correct artwork.
To add artwork to an entire album, click on the "Album" heading in the iTunes window. This will organize the songs according to the album. Hold down the shift key and select the first and last songs of the album to highlight them. Then click on "Get Info" in the "File" menu, click the check box next to the box that reads "Artwork." This will group the album together, and all you need to do is find the artwork for one track of the album and the entire album will be added.
Make sure the computer is connected to the Internet and click "Start," then "All Programs," then "Windows Media Player."
Click on the "Library" tab. Find the MP3 you would like to fix.
Right-click on the MP3, then click "Find Album Info." If the correct album art is available, select it to assign the art to that album. Repeat these steps as necessary with other MP3s.