It is not immediately obvious how to add a photo to be displayed in Gmail for a custom, non-Gmail email address. Here is how you can do it as of January 2019:
- Click your photo (or your initial) in the upper right corner of the screen.
- Click the blue Google Account button.
- From the sidebar menu, select Personal info.
- In the Contact info section, click the Email box.
- Click the Alternate emails box.
- Add your custom email address.
Your photo should start appearing in Gmail conversations within a couple of hours!