With the help of free online image converter you can get the result you need – in available MNG format settings you can set up a color depth from 1 up to 64 bit and adjust additional conversion parameters for indexed colors (8 bit and less).
BMP converter enables to convert various graphic formats, such as: JPG to MNG, GIF to MNG, PDF to MNG and others.
Important! Partly you cannot see settings in “MNG optional settings” field because of their unavailability for the combination given. By changing color depth for indexed colors (8 bit and less) you will get access to more flexible index formats settings.