I might not have understood your question completely, but if your asset is an image, you can go to the Element tab and select the image from there (in the section named Background). In case of other assets, you possibly can’t.
We'd like to be able to have an easier drag at least into this field, as right now you have to re-select from the open dialog. If it is already in the Resources Library it wont duplicate the asset though.
The approach I'd take in these cases is to use a Rectangle element and set its Inner HTML to what you want. Inner HTML is an animatable property. (You can set src of assets to "${resourcesFolderName}/assetname.mp4" so that it gets referenced correctly).