Get GIMP, an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring. It has many capabilities. It will take a little bit of time to understand and become comfortable.  Open the desired image file in GIMP and in the "file" command of the menu bar, export, not save, to another image file name ending in .bmp and, when prompted, use "advance" save settings to save as 24-bit *.bmp.