You can display your profile picture or your company logo to your email recipients who use Gmail by creating a Google account on your custom email address.
1. Follow this link, and click Create an account:
2. Fill in your first and last name. Then, instead of entering your Gmail address, click Use my current email address instead and enter your custom email address. Create a password, confirm it, and click Next:
3. Verify your email address by entering the verification code. Then, proceed with completing your account verification.
4. Finally, click on the picture icon in the top-right corner and upload the preferred profile picture by following these guidelines: Add your logo to Google Workspace:
That's it! Once you complete these steps, all the recipients using Gmail will see this new logo for the emails sent by you. By the way, it will be visible on Gmail for both new and previously sent emails.