Why does Mac Mail keep removing the hyperlink from my signiture image?

Hi,

I have no problem adding an image to my email signature, but when I try to create a hyperlink from it, I follow the process, and everything looks fine,

1. Highlight the image
2. use control + K shortcut
3. Add intended destination URL
4. Click OK.

But once completed I send a test email, and the hyperlink isn't working, I go back to check the signature, and the hyperlink has disappeared.

Why is this? can anyone help?

I'm using a mac book pro operating on OSX Majave 10.14.6