Smart Objects + Masks
This video shows what to do if you want to mask part of an image to be imported as a smart object:
1. Create the mask on the file that you want to remove part of
2. Open the file in which you want the image with the mask
3. File - Place Embedded [the file with the mask appears on the canvas and as a separate layer]
4. If you want to edit the mask, click on the layer containing the masked image, it'll open as a separate file, edit the mask in it, save and go back to the file where it's placed as smart object and it'll be updated.