how do I add a link to an image on mail?

Assuming you're referring to Mail.app, it's pretty easy:

1) Insert the image into your email - either via copy/paste, or via the Add Image button in the message's title bar

2) Select the image

3) Use Edit -> Add Link... and enter the target URL

Note that you won't be able to follow the link while you're authoring the message, but it will react appropriately for recipients.

Other mail apps may act differently.

In Mojave 10.14.6, you can select the image, add the link, and have confirmation that the image has a link associated with it.

With Catalina, you can still click on the image, add the link, but there is no confirmation of a link on that image until the email is sent, and the recipient sees the link hand on the image. Even then, double-clicking anywhere on the image will not automatically open the link assigned to the image. The recipient must press the option key, and click on the image to directly open the link in the default browser.

Do not expect an attached image in Mail to behave like a selected file or folder icon in the Finder. Click once on it. There may be little to no difference in its appearance, but if the Edit menu : Add Link… menu item is displayed, then you have obviously selected the image, and can apply the link.