Hi Edgars, thank you for your question.
In www.symbol.army we do not support to load custom background maps. However, have you tested www.map.army? This web application provides the functions to draw, edit and share military maps.
You can also load custom maps into the web application. Please check the following video on how to load custom maps into the application:
https://youtu.be/jV0iaUTjQlk.