Add clickable link on an image

If you’re trying to link the image to the page in the post editor you can click on the image and then click the link icon to set the target.

You can do the same thing in HTML (in the editor in source view), in a plugin, or in a theme like this:

<a href="https://wordpress.stackexchange.com/link/to/your/page">
  <img src="/link/to/your/image" />
</a>