You can add images/documents at the time you create the asset OR at any time by editing the asset.* You can learn how to edit an asset here.
Images can be added towards the bottom of the upload/edit screens where there are sections for “IMAGES” and “DOCUMENTS”. If you expand either section you can click on the (+) and upload photos/documents. In the documents section there are multiple categories to choose from, one of which is floor plan. Screenshot below:
*You can also add photos during bulk upload. You can learn how to do that here.