The S25 can accept a 97x26 pixel, 256 color, uncompressed bmp file. Of course the S25 is capable of displaying only 4 colors (Red, Green, Blue and White), so if your 256-format bmp actually contains more than 4 colors it will be automatically converted. The conversion is very crude, it does not support any dithering so you should either do it yourself (using a program like Photoshop), or design the logo in 4 colors. Either way, be sure to upload an image containing the 4 colors that the S25 supports or the result will be unexpectable. I have some logos ready for you to download and use. I have created myself all those you can see a preview of. If you have some of your own, why not send them over.
Here are the 3 Greek GSM networks:
Hehe, my favorite Greek team, Panathinaikos! :
Here is one of my personal logos :
Logos submitted by visitors of the site :
Aston Martin by Oliver Gee
438 other logos :
S 25 Bitmap Editor :
The bitmap editor is a simple paint program that is specially designed to produce S 25 logos. Unfortunately the program is in German, but I don' t think you will have any trouble using it.