I looked at the format of that file some time ago and here is what I think about it.
It contains a sequence of thumbnails in PNG format. The .dat file contains a RIM's own header (first 37 Byte) and footer (last 41 Byte).
After the header begins the PNG signature: 89h 50h 4Eh 47h 0Dh 0Ah 1Ah 0Ah
Each png contains 3 CHUNKs: IHDR, IDAT, IEND
The end of the PNG file is right after the IEND Chunk: AEh 42h 60h 82h
after it follwos again the header and the signature of the next png thumbnail.
So it should be quite easy to write a parser for it to extract all thumbnails (or one specific). I don't know what kind of information is contained in RIM's header, but the most important would be the file name which is readable.
If you write a parser for it, I would be happy to see your code here