well,
I tried playing with the "ipa" file. there's a "game.sp" file inside. which is were all the graphics seems to be located. it's 11 megs. looking inside revelas that this file actually contains multiple other files. primarily - PNG images. I was able to extract 260 PNG files from there. but that's only about 3 megabytes. what's with the other eight? those 260 PNGs indeed contain some ingame graphics. but most of the level layouts and most importantly SPRITES are not there. to make things worse, a good half of those 260 PNGs is somehow corrupted. like there is something preventing the file from decompressing correctly. you can see a part of the image and the rest is a mess, though sometimes you can clearly see the remainder of the image, but the colors are all wrong or pixels positions are shifted. I assume that whatever put those PNGs together in one file, inserts some kind of delimiter or other service information in the resulting "sp"-file., so that simply cutting that file to pieces won't give you the correct PNGs all the time. sometime you'll also get those "markers" or whatever they are and they'll screw the whole thing up.
Strange thing is that some sprites are among the 260 after all. there's some for Grim Reaper (corrupted unfortunately) and some other minor stuff. This makes me think that all the graphics are probably stored as PNGs. but the fact that there are only 260 in plain sight might suggest, there might be some kind of storage optimization (like storing a single header for multiple files). anyway this all still requires lots of analysis.
If anyone has any information on this - please contribute. (any rom hacking/reverse engineering experts out there?)