The manual said:
bmap_createblack(var width, var height, var format): BMAP*
Creates a bmap at runtime. bmap_create loads the content from an image file or from a cutout of an image , bmap_createblack creates a black bmap with given dimensions that can later be filled with a color. Objects created this way are automatically deleted on closing the engine, or by calling the ptr_remove function.

(and i already tryed this few month before)
If you delete the Panel by ptr_remove, the BMAP (crated by bmap_create(); also will be removed by engine)