Help with Photoshop Elements - Smart Object Thumbnail

I’ve dowloaded several PSD files from Envato and opened them in Photoshop Elements 2020 (ver 18). In the layers panel is where I supposedly can add my own photo to the “image here” layer (where smart object thumbnail is). Simple as that and all the features the author created will be applied to my imported photo. Thing is - I can’t seem to import my own photo. Please help. Thanks.