When designing a website, I look again at the latest recommendations for image size for websites, at the best pixels per inch (PPI). This article from PhotoshopEssentials.com (dated only 2013) says, “there’s simply no such thing as a standard web or screen resolution, if your images are destined for the web, you don’t need to worry about image resolution at all!”
The old standard resolution of 72 pixels per inch dates from 1984 with the first Macintosh. Today’s screens are more like 100-150 ppi; for retinal displays, 300 ppi. Each monitor will show the pixels per inch that it is designed to show or a smaller number if the settings have been changed. Resolution only applies to print images.
A file with a resolution of 72 will be very small when printed and cannot be enlarged with a satisfactory result. The top dialogue box in Adobe’s Photoshop programs applies to images on the screen. The lower box, where the resolution and size in inches appears, applies only to printed images. Photograph set to 200 ppi will still be 200 ppi on screen whether the resolution is set to 72 or 300, and the file size will be the same. But the photo will print very small if set at 72.