Newsletter email not displaying images

In Mailpoet ver. 2.7.10, dispatching the newsletter email via sendgrid — some of the recipients receive the email with blank spaces for the header image and for the blog post image.

Not sure whether this has anything to do with the client email software, but… of of my test recipients, using MS Outlook displays the images properly; another, using gmail images are missing: when viewed in Firefox, the header image is visible but the post image is not, and when viewed in Opera both are not visible.

Actual recipients, not using gmail, complain about not seeing images.

How do I fix this problem? Thank you.