With the help of free online image converter you can get the result you need – in available PNG 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).
PNG converter enables to convert various graphic formats, such as: JPG to, GIF to PNG, BMP to PNG and others.
Important! Partly you cannot see settings in “PNG 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.