Google has made it a little easier to add images to Gmail signatures when you have those images stored in Google Drive.
In Gmail, click on the cog wheel icon then choose Settings > General. Scroll down to the ‘Signature’ section.
When you click the Insert image icon in the signature editor toolbar, you’ll see the option to add an image from Google Drive.
Once it’s in the editor, you can click on the selected image to change its size:
Once you have your signature the way you like it, save the settings and you’re done.
Note: In order for this to work in administered Google accounts Gmail, admins will need to enable external sharing of Drive files in Admin console by using Drive > Sharing settings > Sharing options.