deniska, I tested different sized PNG/JPEG POIs and directions and everything seems to work (thanks for the no lat/lon logic).
Except one thing - missing tiles (thought this may be due to IDXoffset=0 first/last tile in the files, but the second example seems to suggest otherwise):
2x002005.png is not displayed from this (London tube) map:
http://homepage.mac.com/f.levin/psp/GPSFS2error.zip
and
http://homepage.mac.com/f.levin/psp/GPSFS2error2.zip
has one tile at the top zoom level missing and two at the base level.
GPSFS_Extracted in both to illustrate that they do extract ok.
Also, dummy files are now created for small maps (eg. the error2 one above) and seems to work ok.
Bookmarks