To add a photo to your comic, drag an image from the photo browser and drop it on your comic.
The image is sized so that it fits reasonably on the page.
When you are dragging an image over a panel the panel outline will be drawn with an orange color indicating that it is ready to receive the image.
Dropping the image on the highlighted panel will add the image to the page so that it fits exactly within the panel. Any overspill from the image due to making it fit is clipped by the panel.
See Working with Panels for more information.
Once you start editing other areas of your page you'll notice that you can't directly interact with the image in the panel - you'll select the panel. To select the image inside the panel double-click on the panel and the image will become selected.
You can quickly add a number of photos from an album or folder, see Autoflowing Images for more information.