If you're having problems with Home Inventory 4 for Mac, don't panic. We'll have you back up and running in no time.
Yes, drag & drop works, but there are other ways as well. You can check one of the “How To” videos.
First, select your inventory item, then open the tab called “Documents” (it should be selected by default). Next, simply drag a photo from your Finder window to the “Inventory Item Documents” list.