Increase contrast using a subtle inner/outer shadow, typically used with user-provided images that may or may not have a high contrast relative to the background.
It's best to use
.shadow-contrast-inset on images. Notice how it affects light areas only.
.shadow-contrast may feel like adding a subtle blur around areas where the image is only slightly darker than the background. It's best when the background is darker than the image (or whatever foreground element is used).