Depending on how you created the signature (an HTML TB message file or direct HTML entry), you want to surround the code for the image source with the code for the link. This is easily done if you define the link when you insert the image into the HTML, as shown in the picture in [https://support.mozilla.org/en-US/kb/signatures#w_including-image-files-in-signatures this section]. See the 'Image as a Link' section [https://www.w3schools.com/html/html_images.asp here].