IIRC, you used to be able to do this using the messenger api with this code:
code:
oMessenger.MyPropery(2) = <location of file>
with oMessenger beeing and instance of the MessengerAPI
not sure if this code is still compatable with the newer versions of messenger.
Also, the images stored in the tiles directory are just png's renamed to dat's. But to use the directory to add a display picture you have to edit the map.dat file, which is encrypted. So the api method is the best way to change the display picture (assuming it still works)
Hope this helps, and please post back here if you're having any problems.