You can make your own logo to be placed on the idle screen. Follow these specifications to create it.
Format: save the logo as BMP in 256-colors (8-bit) and without colorspace info.
About colorspace info: by default, Gimp includes this info, resulting in a malformed logo on the phone. During export to BMP, deselect this option (under compatibility options). MS Paint never saves colorspace info so no specific settings are required.
Do not save in 16-color (4-bit), especially with MS Paint: you will lose the darker shade of gray (404040) because it is not part of the default 4-bit palette.
If the dimensions are larger, the phone will automatically downsize, although it is recommended to use the proper dimensions in order to avoid quality loss.
Colors: 4 colors.
These are (RGB values): black (000000), white (FFFFFF), and two shades of gray (404040 and 808080).