I just uploaded #2215 that should add support for uploading custom icons. (Settings->More Options)
Its a first release, and needs some more fine tuning, but hope it works
(Gui is not translated yet also)
The icons are stored in the database, and are cached and also written to the www/images folder if they do not exists,
so when you make a backup of your database, this should let you keep your custom icons as well.
You need to supply a ZIP file, the contents is described in the updated manual.
It is possible to add multiple icons in the zip file.
Currently only supported for On/Off switches (as it is now), and probably later for other types as well
Also the dropdown to select the icon has to be rebuild as it is now a long list, when you have 50 icons its very long

/Rob