Actually the banner images for Even Weirder Worlds, have always been intermittently missing on my 10.4.10 system (they are almost always not visible, but occasionally are).
Yeah, it is merely the banner images that are giving me fits. I don't have a lot of time to look into it now, but if nobody else is having a problem, I'm okay dealing with it for now.
Have you checked that your pallette is correct? I remember something in the modmaker's guide saying banners need a specific pallette of colors to display.
If that's the problem, you can solve it by duplicating the banner file from the main game, then pasting your image into it.
If the palette is wrong, you'll just get wrong colors in the menu. If the image is not loading at all, the problem must have something to do with the file format. You must save your .bmp images in uncompressed 8-bit. (This also means all banners should be identical 29160 bytes in file size)