From the [formatting help page](https://mathoverflow.net/help/formatting):

> Images can be added primarily by using the the editor toolbar button ![insert image toolbar button](https://i.sstatic.net/C3zkF.png). This brings up a special interface that allows you to upload an image online (through the imgur hosting service) through us - even from your clipboard. Alternatively, it can be input similarly to adding a link:

> * HTML `<img src="http://example.com/img.jpg">`
> * Markdown `![sample image](http://example.com/img.jpg)`