Hi there MacGraphics Services,
If images aren't appearing as expected in your email signatures, let's first ensure your Mac is fully updated to the latest version of macOS. This installs feature improvements and bug fixes that can resolve behavior like this. These steps from Update macOS on Mac show how to do this:
Before installing new software, it's a good idea to back up your Mac.
1. From the Apple menu in the corner of your screen:
• Choose System Settings. Then click General in the sidebar, and Software Update on the right.
• Or choose System Preferences. Then click Software Update.
2. If Software Update finds new software, click the Update Now or Upgrade Now button to install it.

3. Enter your administrator password, if asked. During installation, your Mac might restart, and you might see a progress bar or blank screen several times.
If Software Update says that your Mac is up to date, then no new software is currently available for your Mac model. Software Update shows only updates and upgrades that are compatible with your Mac.
Once your Mac is updated, next ensure you are following these steps to create a signature in Mail: Create and use email signatures in Mail on Mac
Take care!