A quick method would be to create a grid 5 x 3 and increase the outline thickness to give suitable spacings between each photo. Scale the grid to match your postcard size and align to the page then change its colour to white.

Next, open object manager and lock the editability of the grid layer, add a new layer and make sure this new layer is below the grid layer. Now when you drag your photos into the appropriate postion the grid will act as a white border ensuring all photos are of equal size.

Sark