You may easy fill existed layers with images from a folder.
Select from menu File - Load Layers From Folder...

This will ask you for a folder with image files and will insert these pictures into existed layers.

When you select this menu item you'll get Open Folder dialogue with additional options:

Cycling Reuse to Fill All Layers - This will reuse images over and over until all layers will be filled.
 
Fill Empty Layers Only - It will fills only layers with placeholder images. Layers that are already contain a real images will not be changed.

Fill Selected Layers Only - Only selected layers will be filled.

With Subfolders - Program will search for images in the folder that you select and it's subfolders.