I am new to rockbox, and had difficulty finding information on extracting album art when using OS X. If this is helpful to people, maybe someone more experienced can post this somewhere in the wiki.
I did this using OS X 10.5.8 and iTunes 9.
In order to extract id3 album art to the album folder
1. Download and install Doug's Scripts for iTunes - Save Album Art to Album Folder - from here
http://dougscripts.com/itunes/scripts/ss.php?sp=savealbumart2. Highlight the albums or tracks you would like to extract from. You can chose to extract from track art to album folder or album art to album folder. Choose album art, especially for larger libraries.
Voila!
If art is extracted into a form which Rockbox cannot read, you can create an automator workflow to search for .png, etc within your music folders and convert them to .jpeg or .bmp.
To create the workflow, open Automator, choose Custom.
1. Drag "Find Finder Items" into the workflow.
2. Choose the folder where you store your music in the Find Finder dialogue.
3. Choose "Extension" "is equal to" "png" or other undesirable image extensions.
4. Drag "Change Type of Images" into the workflow.
5. Choose To Type: "JPEG" or "BMP" whichever you prefer.
6. Save the file and run it.
When you create the workflow, you are able to choose whether you would like to save the original image file, I chose "no" for this, you can choose to save it if you would like. The program will automatically add this step if you decide to do this.