How about making one (or more) big imagefiles and have an xml or database-entries with coordinates. That must be more efficient than caching 168 small thumbs?
Some devlopers are working very hard to bring the next generation of GUI engine based on XAML to reality. Hopefully this will be ready for the 0.3.0 release and obviously you can expect the default 4:3 skin to look like 3DMP from Vanessa.