To add a photo to your project once it is uploaded to the editor, go to the "Photos" tab in the bottom right. Here you can upload new images, or choose from a gallery in your account.
There are two ways to add the image to the layout. You can either click and drag the image into the grey image area in the layout, or you can click the image within the gallery on the right-hand side to add it to the highlighted layout (there will be a small blue border around the highlighted page).
Clicking and dragging the image from the gallery into the desired layout will insert the image in the layout you choose. First, click the image in the gallery, then drag it to hover over the image area within the layout (the highlighted image area will show a darker grey), and let go. The image will be added to the image area in the layout.
Clicking the image in the gallery will insert the image into the highlighted layout or the next blank layout.
An image will appear slightly greyed out in the gallery with a check mark over it if it has already been added to the project.
To remove an image from a layout, hover over the image in the layout and click the pencil icon, then click the red "delete" button in the bottom left corner of the image editor screen. The image will be removed from the layout, but can still be found in your gallery on the right-hand side of the editor.