If you have images and you are showing them on the web using an <img>
tag, you must ALWAYS include alt text.
If you are writing markdown, you have no excuse not to add alt text.

Doing this is lazy. You're better than that.

Put your decorative images in your css, plz. But if you have to put it in your markup, you can exclude the alt tag. And I am talking decorative. Like background image of repeating pictures of Tom Hardy hugging a puppy, has nothing to do with your website, decorative.
<figure>
<iframe aria-labelledby="embed-caption"></iframe>
<figcaption id="embed-caption">This am what the thing are</figcaption>
</figure>
YES you absolutely can!
<figure>
<img src="…" alt="A rad wolf">
<figcaption id="embed-caption">Look how dope his shades are; are you even kidding me with the radness of this wolf</figcaption>
</figure>
a11y is important. It's so important COMPANIES HAVE BEEN SUED OVER IT. Please add alt text and figcaptions to your images and embedded files.