From Forum Nokia Wiki

Bitmap converter that takes one or more Windows bitmap files (.bmp) and generates a single Symbian OS multi-bitmap file (.mbm).
This Converter is used to convert bitmaps between Symbian OS and Windows bitmap formats. Symbian OS bitmaps (mbm) may be produced as either a font store file or a ROM image file, which does not use any RAM when being accessed. The conversion process specifies the number of bits per pixel for the converted bitmaps and whether they should be colour or grey-scale.