For converting back and forth between the image and source you can use xxd. In a Windows environment you can use this command in the Linux Subsystem installing one of the versions available (i.e. Ubuntu).

xxd -i input_image.bmp output.hpp

xxd -r -p input.hpp output_image.bmp

There is a special file that contains all the firmware icons firmware\application\bitmap.hpp but this file should not be edited directly, instead, new icons should be created in firmware\graphics and then run firmware\tools\make_bitmap.py <DIRECTORY> to create bitmap.hpp.

For operating that Python3 script, Pillow library is required: pip install pillow

From bash, the command that creates and moves the bitmap file is:

python3 make_bitmap.py ../graphics/ && mv bitmap.hpp ../application/bitmap.hpp

The current icons were designed on Piskelapp. To create or update icons, the current sources (16x16 or 8x8) can be used.