As to the Irfanview tutorial specifically: I'd add that you might as well "decrease color depth" as well (or do that together with grayscaling). By decreasing it to 16 colors, I reduced the file size from over 1 Mb to less than 250 kb. With no damage to the picture quality (for these purposes) that I could see.
I like to put a lot of different logos in there and let the software choose one at random for closing off and starting up. Would there be a firmware limit tot the amount of files you should put in there?