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Photos should be 2400px by 1600px. This the standard Photo Shelter "large" size. (The height may vary slightly. UW's ratio puts the height at 1597px for large and WSB's ratio puts it at 1661px for large.) 
A note on hero photos: the main content of the photo should be vertically centered in the photo because at large screens (when the photo gets really wide), we crop the height of the photo so it doesn't take up tons of vertical space. So if you're photo has people in it, make sure their heads are vertically centered.
Directory Photos
These are styled into a circle via CSS, so the uploaded photo must be a square. The square should be 512 x 512px.
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