Here are the steps:
Start with adding a new page that will serve as a cover. Click Add Page in the bottom left corner.
Now open the Photo tab and click Add Collage. Select a few photos or upload the whole folder.
Adjust the layout. Set the number of photos per row and spacing between them. You can also add a shadow and an outline. Position the collage on the page as you like. Click Finish to create the collage.
Here is the created customised calendar with the photos from previous pages: