If you see a broken image in your signature, this is due to a bug in Gmail for non-Chrome browsers. To solve this issue, you need to upload your image on a service other than Google Drive(the one offered by default in Gmail) and add it via the URL in Gmail settings.
Go to Gmail’s General Settings
Find Signature in the General tab and add your text signature
Tap on the button to add an image
Upload your image to a service providing you a direct link to the image(important: don’t use Gmail’s own uploader) and paste the link in the popup
Select the right size for the image
Enjoy your shiny new signature that will work across browsers and not just in Chrome🎉