Making a functional and hyperlinked email signature in Apple Mail is very easy, and very limited. You can easily hyperlink text, you can’t hyperlink images (as of 2020). There is always a workaround.
For text; simply highlight it, right-click (control-click) and look for ‘Add Link’ –> paste/edit the link.
The same ‘can’ be done if you paste an image and if you use Mail –> Edit –> Add Link (then add the link), you’ll find it doesn’t. Even if you edit the HTML of the signature file, you’ll see the hyperlink in there, although you’ll still find the image won’t act as a hyperlink when viewed in an email.
Simple solution is to use images just as an image and where you need to hyperlink, use text.
Alternatively you can use HTML email signature editors like HTML Email Signature – Mail from the App Store https://www.element26.net/products/shop/