Masthead images, which only appear on the homepage of a site, should be 1920 x 1080. Make sure the subjects of this photo are vertically centered, as these photos will resize vertically and might awkwardly crop subjects out of picture.
In order to ensure that photos are sized properly when web pages are loaded on any device, photos in a feature block or photo-slider should be 700 x 394. These photos will automatically resize to fit the content area of the requesting device, be it desktop, smartphone or tablet.
Any other photos (inline) should be 300 pixels wide.