Technical details…
Apps offering the possibility to add custom icons are using a data folder where the app can store the file. It’s really cumbersome.
I also would prefer a solution where the app can provide a list of icons for a device from which the user can select one.
It was already requested. I can ask again…
Edit: asked again 
[FeatureRequest] Select you own icon for a device - #97 by RonnyW
