How can I add a picture to an mp3 song in Windows 7?

You want to add the album art cover to the song? If you are using iTunes just select the song(s) you want to use the picture with and drag and drop to the small square at the bottom of the left side menu. Make sure it says Selected Item and not Now playing, to change from one to another just click on the title of the square. After you drop the picture there the files will automatically update. You can also select all the items you want to update and then right click and select Get Info, from there you will see a field that says Artwork, you can drag and drop the album cover there or double click the field and browse to the file location.
If you are using Winamp you will have to select the items you want to update and open the mp3 ID tags in a similar way to the Get Info procedura in iTunes.
In Windows media Player you will have to drag and drop over to where the Album cover should be.
If you are using a different media player, let us know.
Hope it helps.