Dolphin already has a 'Tag' icon. However, when a user adds a tag to the places panel (using for example 'tags:NameOfTag/' on icon is shown. Yet Dolphin has a tag icon already which should be applied (it is not even manually available to apply at present).
This works properly, as expected if you drag the tag to the Places panel or add it with right-click on tag > Add to Places. The only time it doesn't work is if you're manually creating a places entry from scratch via right-click on Places panel header > Add Entry... , because with this method, you're required to supply your own icon.