About .bmp graphic file:

BitMaPped graphic - standard, non-compressed bit-mapped graphics format used in the Windows.
BMP files store graphics in a format called device-independent bitmap (DIB).

Bitmap images, also called raster images, use a grid (the bitmap or raster) of small squares known as pixels to represent images. Each pixel is assigned a specific location and color value.

Help how to open BMP files:

Use any graphic editor/viewer (XnView, Paintbrush) available to open or view png file format. Supported by various graphic editor/viewer.

How to convert file with extension BMP:

Use any graphic editor/viewer (AcdSee, Adobe Media Encoder) to convert formats.