For most designs the main image or photo on your site is called a slide. You can change this from the "Slide" option on the Quick Add menu> "Slideshow/Main Image Settings"
If it's the first time you've added a slide you'll need to go to Design> Main Image and choose whether you would like a single image or a slideshow.
You can also change the slide from the Main Image menu.
The system will automatically crop your image to fit your particular design, but if you don't like the way it crops the photo you'll need to crop it yourself using software like Photoshop or online image editors like Picnik or Aviary.
Need more help? Try this tutorial!