Many companies use their logo as a visible indicator to help email recipients recognize their messages. This image or favicon shows up when someone sees your message in their email inbox.
For example like this:
In order to add such an image also for newsletters sent from a custom domain via ESP, you can proceed as follows based on your email domain's service provider.
For Gmail, you can simply follow the steps outlined here to change the image.
For other email providers, that do not have the option to change the image in their profile settings, you can create a Brand Indicators for Message Identification (BIMI) Record that will contain your image.
Please see the steps described here on how such a record can be added.
Note, that you would also need to generate a VMC certificate confirming your rights to use the given logo so that it's displayed in the inbox of the most common email providers. (Here is some more information about this from Gmail)
After getting this certificate, it has to be uploaded to your domain (similarly to the logo itself) and added to the BIMI DNS record.