Why use box-shadow

If it's just a single image there's usually not that big of an advantage to using a box shadow. It will generally be quicker to add the shadow using CSS and quicker to edit/tweak ongoing without having to go back and export a new image. But, like I said, with only a single image, the advantages gained are not as great.

If you're dealing with an entire page of images, or talking about a side-wide style applied to all of your images, the advantages are huge. You'll be able to quickly apply the shadow via css and style all of your images at once. If you want to tweak the shadow down the road, that's easily accomplished by editing one single line of css rather than going back to photoshop and editing/exporting hundreds of images again.

Box shadows also do not affect the box model and page layout because they are not considered in the image's total width or height.

If you're aligning your images to a grid, for example, you might have to compensate for the shadow space if it's embedded in the image.

If you ever want to change the shadow, you don't need to fire up Photoshop again: just change it in the CSS.