I thought the maximum size was 1024 *1024
Im very surprised to see one at 4608, especially as thats not a power of 2, and to my knowledge all the bmps must have sides that are 2^X eg 64, 128,256,1024 (2048,4096)
They dont have to be square though, thats a point worth noting.
AFAIK you're right about the 256, 1024 etc. I've tried it with other sizes and it refused to recognise them.
I have tried making a highly detailed bitmap (2048x2048) but I kept getting memory errors, it must be pretty hungry on the RAM. It's not uncommon to enlarge a texture to 3072x3072 for repainting in detail, but you resize it back to 1024x1024 before flying.
The bitmaps don't have to be square but they do need to keep to standard sizes - 256x512, 128x1024, etc.
Gryshnak