Photos must be sized appropriately for different uses. This helps avoid an awkward cropping after the photo is loaded into the site (and makes your pages look great).
Tip: Select an image focal point in the CMS to avoid the most important part of your image being cropped by the screen.
1/2 Div Container
1/3 Div Container, 1/4 Div Container
600px width max, 600px height max