I struggled getting my custom icons (previously working on OH1) to work in OH2 as desired.
What confused me was the separate folders for classic and basic UI.
However, whenever I placed icons in basic UI folder /etc/openhab2/icons/basic it did not work (I am using basic UI only).
As soon as I placed it in classic, it does work (make sure that you place the “non-state” icon there as well: espresso.png for espresso-on.png / espresso-off-png).
I agree. My icons are in the classic folder, but are most definitely a fully customised set. Perhaps there is a simpler way to manage custom icons. A ‘custom’ folder which overrides the classic folder if icons are the same name? shrug
Icon sets require some meta-information (name, label, what formats does it support, etc), a plain folder simply cannot provide this.
We would need to build a bundle for this (see https://www.eclipse.org/smarthome/documentation/features/icons.html#icon-sets), so that another icon set is correctly registered.
Note that there are quite some requirements for an icon set, e.g. it should have an icon for every ESH channel category available and it should also have different versions for different item states - so if someone really has packed all that into a personal icon set, I would suggest to share it with the community and contribute it as a then-non-custom iconset
For all other cases, it imho makes sense to use the existing icon sets as a fallback and only “override” it with your custom versions, where applicable.