In more detail
Designers and developers use them to lay out pages, templates and prototypes without waiting for photography. A good one shows its pixel dimensions, such as 800 × 450, in a neutral colour so layout problems are easy to spot and nobody mistakes it for final art. It can be a tiny generated SVG, a solid-colour PNG, or a blurred low-quality preview shown while the real image loads.
Why it matters
Giving the placeholder the same size and Aspect ratio as the final image reserves the space, so the page does not jump when the real picture arrives (a layout shift that search engines measure). Placeholders that slip into production look broken, so search for them before launch. Avoid using random stock or found photos as stand-ins, which can end up published without a licence.
Example
A self-describing 800 × 450 placeholder as an SVG of a few hundred bytes:
<svg xmlns="http://www.w3.org/2000/svg" width="800" height="450">
<rect width="100%" height="100%" fill="#cccccc"/>
<text x="50%" y="50%" text-anchor="middle" dominant-baseline="middle"
font-size="40" fill="#555555">800 × 450</text>
</svg>
The grey label on the grey background has a contrast ratio of 4.64:1.