What file types are you working with? If they are WAV files, it is somewhat difficult to replace artwork.
If you are working with AIFF, Apple Lossless, AAC, iTunes purchased or MP3, you can look at the properties of the file (Get Info) and on the Artwork tab you can add, delete and drag the artwork to order it. If there are multiple graphics files attached to the song, the leftmost one is the one that will be displayed.
When you drag new artwork onto existing artwork, it will often show the new artwork, but as soon as you select another song and then go back and select it again, the original artwork will show. Both versions are there, but the one you just added is second in order and will not show until you drag it to the primary position or delete the original artwork on the Artwork tab.
Steve