Step 08: Fill the shapes with photos to finish your Photoshop collage
Now it’s time to fill your shapes with your image to create the photo grid! First, you might want to clean up the preview and hide the grid view by going to View > Clear Guides (don’t worry, you can leave it on if you like, and it won’t affect your printed or exported file).
Now, open a folder on your computer with the photos you want to use in your photo grid. Then, select one of the shape layers and drag/drop your photo to the preview window. Press and hold the Opt (Mac) or Alt (Win) key and hover in-between the shape layer and the image (just below the image you just added), and click when you see the icon change. This will create a clipping mask, so the image you added is only visible within the confines of the shape layer. Now you can resize the image layer (select the layer and go to Edit > Free Transform) to fit better within the shape.
Continue adding your images above the shape layers and clip/transform them until your entire photo grid is filled.