To store items in the objects, go into live mode and drag any inventory-compatible item onto the storage object. You can drag directly from your sim’s inventory or from an object already placed in the game!
Photos may turn black when stored in these containers for an extended period of time. Make sure to upload your photos to the gallery if you plan on storing them in these bins just in case! This is an EA bug and I can’t change it. I will continue to work towards a solution, promise!
Update 08/09/21: I have found a solution to this photo bug and will be releasing a new storage set specifically for photos. Any photos stored in this set as it currently is will turn black if the storage items are double-inventoried (placed into your sim’s inventory).