Signature issue on outlook

My boyfriend uses outlook to see his Gmail emails. He wants to add a signature with his company logo, but anytime we click the picture button nothing happens. From all the tutorials it should be as simple as clicking that and uploading a file from your computer but that’s not working for us. Is it because it’s a Gmail not an outlook email address? Or what can I do to fix it, because he wants to use the outlook app on his computer, not use Gmail on the web.