Have you changed your signature's image (logo, personal image, or banner) and detected that image isn't automatically updated in Gmail? The main reason for that is your web browser store or "cache" images as a way to improve your experience. Clearing your web browser's cache removes these stored assets, which may help resolve issues with the image updating in your email signature.
To clear your cache in Google Chrome, click the menu and select More tools. Then click Clear browsing data. Make sure that the Cached image and files box is checked. See more details on clearing your cache in Chrome here.
To clear your cache in Mozilla Firefox, go to your Preferences and then find the Advanced panel. From there, click Clear Now in the Cached Web Content section. See more details on clearing your cache in Firefox here.