You just need to run consolidate library function from menu, starting from version 10.75 if you are changing tags from another program.
You also must know how does iTunes works:
iTunes 7 will automatically search the iTunes Music Store for artwork, and if any is available it will add it to your files. The problem is that iTunes doesn’t add the artwork to the actual ID3 tag within each file. Thus, the artwork is visible only to iTunes.
The workaround is a manual one, but it’s quick and easy—and it works! The only step the author neglected to mention is that you have to start one of the album tracks playing so the artwork will appear. Also, this works only with entire albums, not individual tracks. Thanks, Sheppy! — Rick Broida
Read more about it at: