Can you share a document @stilmi? It's unclear what you are doing. You cannot place an image on a symbol unless you mean that you have dragged it from the resources folder. Also you cannot drag something out of a symbol. You can drag it around within the symbol but not out of it.
The symbol has a background until you place something inside it. Is this what you mean? You have to be inside the symbol (double click) to put something in it.