I've been trying some simple tests to understand how the PNG's are stored in the P3T file, the header keeps the location of the beginning of the file and then list the size the file. The next file begins after the Hex String is 0'd off.
the first 4 hex bytes define what the icon is (MUSIC(D9), PHOTO(D1), GAME, etc.). It appears that the png is converted into a physical representation.
For those interested in see what I have just download the PNG's and xml below, this will simply your task.
(BTW Jpg's aren't converted.)