We automatically scan your newsletter's content for images to use in the Twitter Card, but if you have multiple images, you can specify which to use for the Twitter Card in the source code for your newsletter.
Click the < > button in the content editor toolbar. Then find the image URL you want to designate as the main image, and add the attribute tl:main=“yes" to the img tag. The end result should look something like this:
When you're working with the Gallery Card format and you specify an image as the main image to use for Twitter, add the attribute tl:image=“yes" to each additional image that you'd like to be pulled into your gallery.