Use photos that are wider than 1500px. Publish looks after the rest.

When you publish your post, Publish uploads copies of your photos and hosts them online. Each photo is automatically resized to 5 different sizes, so your post loads fast on every device and screen size.

Publish resizes each photo to these widths:

Each visitor gets the size that fits their screen. For example, if someone views your post on a phone with a screen 500 pixels wide, Publish sends the photo that's 500 pixels wide. This saves data, so your post loads fast on every visitor's device.

Resizing doesn't make a difference you'd notice. Your photos can even look better, because photos that are scaled down from a larger size to fit a small screen often lose sharpness.