Adding a cover image to an asset can only be done in the Web Portal, this will attach to the Asset Card.
Navigate to the Web Portal > Assets > Asset Register > Select the Asset you are wanting to edit.
If there are no images on the asset card, on the left hand side will be an option to upload a photo, you can click here and choose the required file.
If there is a photo already uploaded, you can click on the photos tab in the asset card to click or drop another file to attach it to the asset.
Once uploaded you can click the green edit icon to edit the name, description or tick the box to make it the cover image then save your changes.