Joey did a great job describing the origin and real-world use of the AddProp.ini file in a posting yesterday, and it happens that I ran across a another use today.
Similar situation: You have already generated a .GTX file and it's in use on a hand-held. Now you want to add raster files. You _could_ change the .GTM file to add raster data (as described in the trusty gtvconfig.doc) and then regenerate the .GTX file with GTPack, and copy it onto the SD card. Or... you could simply describe the raster files in AddProp.ini and place it on the hand-held. Here's how.
Suppose your .GTX file on the hand-held is on an SD Card in a folder called "City". Also suppose you have a set of backdrop aerial photography in .ECW raster files in a subfolder of "City" called "raster".
Make a text file called AddProp.ini that looks like this (where "+" means the pipe symbol (which has a hard time on this blog)):
Now copy this file over to the hand-held into the "City" folder.
Now start Pocket GTViewer, open the .GTX file, and the raster icon along the bottom of the display should let you toggle the raster display on and off. Voila!