A number of people are reporting an issue with the latest iTunes 9.2 update, where after installing it the Finder no longer displays album artwork for songs when previewing them in a Finder window. This is a relatively minor bug, but one that seems to happen in Snow Leopard when artwork has been added manually or with third-party tools.
This issue could be because of problems with the iTunes library, or the XML library file that is found alongside the iTunes library itself, which is used for associating library items with other applications, including Apple's iLife suite.
In testing, I added artwork to a song on a system running iTunes 9.1, which appeared properly in the Finder and in iTunes. I then updated it to version 9.2, but the artwork still appeared properly, both in iTunes and in the Finder. This was also the case with newly added songs in iTunes 9.2 for which I have added artwork.
Regardless of my inability to replicate the problem, numerous people have confirmed it to happen; however, it is not a universal problem. This means it is likely an issue with how specific artwork imports are being handled. In looking at potential issues with the library files themselves, there are a couple of possibilities for tackling this problem:
Re-add artwork through iTunes
If you have seen this problem happen for a few items only, then you can try using iTunes to delete and re-add the album artwork to the music files. Hopefully the problem is limited to an improper import of the album images, and using iTunes will rectify the problem.
Rebuild the iTunes library
So far there is no telling if this problem happens because of errors in the iTunes library, but if so one option may be to rebuild the library. This may not be a practical solution for many people; however, if you do not care about some of the organization and metadata that could be lost by rebuilding the library, then you can give it a try.
To do this without harming your current library, launch iTunes and immediately hold the Option key. A window will pop up asking you to create or choose a new library, so click the option to create a new one. Choose a location and name the library, and when you click "Save" the program will open with a blank library. From here you can drag your current iTunes music collection to the iTunes window and the new library will be populated with your music.
From here, try viewing artwork in iTunes and then seeing if it displays in the Finder. If this does not work, you can use the Option key again when iTunes launches to select your old library.