Excerpted from the ReaderWorks site for LIT publishing:

To add cover art you need to use ReaderWorks Publisher.

The recommended image sizes are:

• Cover Image: 510×680 pixels (3:4 aspect ratio)
  
  
• Library Image: 99×132 pixels (3:4 aspect ratio)
  
  
• Cover Page Image (spine): 96×680 pixels
  
  
• PocketPC Cover Image: 480×240 pixels (2:1 aspect ratio)
  
  
• PocketPC Library Image: 90×45 pixels (2:1 aspect ratio)