In Gmail, you can’t add a static file (like the JPG or PNG you can download from your Logomaker account). But there are two ways to get your logo into your email signature. The first way is to create a page on your website specifically for your logo. This page should not have any other design or copy elements on it. Just your logo file. You don’t need to link to it from any other pages on your site. No one will ever visit this page. All you are doing is creating a page from which Gmail can pull your logo each time you send an email.