The menu graphics are more complicated and cumbersome to change in the Neo Geo image compared to the Taito image. In the Taito version, there is a separate .bmp file for each game. In the Neo Geo version, all of the menu graphics have been combined into a single .bmp file: snk6.bmp.
To combine .bmp files into one:
- Place all your .bmp files in a folder together.
- Open cmd.exe from the Start menu.
- Change directory to the folder where you have placed your .bmp files.
- Type this command:
This will create a new file, snk6.bmp, which has concatenated the binary data from all the .bmp files in that folder. After this file is generated, move it into the folder with your game data.
Your .bmp files need to be 16-bit at 320 pixels wide by 240 pixels tall. You need two .bmp files for each game. This provides the ability to do a simple two-frame animation for each game, or you can duplicate your single graphic into a second file for each game and ignore that option.
I haven't yet determined how to properly and easily generate valid .bmp files for this executable. The game board is pretty finicky with its processing of snk6.bmp.
Here is what I know (this is more for the technical nerds who can help me troubleshoot this): It seems that all the .bmp files that make up snk6.bmp need to be BMP Version 3 (Microsoft Windows NT) format, as seems to be indicated when you look at the bitmap header in your hex editor: both 3.1 and NT versions are always 40 bytes long, but only the NT version supports 16-bit color depth. I'm assuming the correct color mode for these images is something like X1 A0 R5 G5 B5.
Here are some helpful resources I've been referencing to try to understand the BMP format:
http://en.wikipedia.org/wiki/BMP_file_format
http://netghost.narod.ru/gff/graphics/summary/micbmp.htm
The easiest solution is to find some image software that will save bitmaps in Windows 3.1/NT era BMP format. I've been trying to cobble together a script to convert a PNG into a 16-bit BMP, but I'm not much of a scripter so it generates some wildly inaccurate colors so far. I'm sure there's an image editor out there that can make saving these in the right format pretty trivial.