Using a custom logo for an email blast can be done by following the steps outlined below.

1) First, you'll want to create a custom email theme.  I've created one using the 'Basic with Logo' theme for demonstration purposes.

2) From here, we'll want to go into 'Theme files' tab and upload an image for the logo we'd like to use.  Make sure to name this file intentionally as we'll be using this name within the HTML template file.

3) We can then open up the 'HTML template' file for our theme and route ourselves to the code that is rendering the logo within the email blast.

We can then remove the following lines of code as we'll want the logo code to run regardless of whether there is a logo uploaded within the website or not.

{% if settings.site.has_meta_image? %} 

{% endif %}

Once this has been done, we'll want to change the {{ settings.site.meta_image_url }} code within line 53 to be whatever the name of the file of new logo is.  In my case, I'll make the following change.

Current: {{ settings.site.meta_image_url }}

Revised: generic_logo.png