How come some of my e-mails don't display images while most e-mails images are displayed?

Under Tools > Trust Center > Automatic Download

[ ] "Don't download pictures automatically in HTML e-mail messages or RSS items"

is unselected.

I looked in the gmail account and the images are there in the message, once downloaded isn't displayed by outlook.

I'm uncertain of the differences between e-mail that displays images and doesn't display images

I tried comparing message headers of a few e-mals that display images and found a few differences such as in Content-Type and Content-Transfer-Encoding, perhaps there are other differences that's causing images not to be displayed?

email where images are not displayed.

Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: base64



e-mails with images seem to have a a MIME-Version and different Content-Type such as

Content-Type: multipart/alternative;
boundary="===============3921082313332266970=="
MIME-Version: 1.0


Content-Type: text/html; charset=us-ascii
Content-Transfer-Encoding: quoted-printable


Installed Microsoft Office 2016 and Outlook still doesn't display images for some e-mails.

Its not just that the Outlook fails to download the complete content/attachment, it is even the issue with rest of the mail clients. I have a Mac as well. I use Apple Mail and Thunderbird on it. And I noticed it there as well. Mail clients tend to save bandwidth and space, therefore avoiding complete download of the contents.
Only, that I used Mail Backup X to backup my mails, i got alerts about all those empty emails. I downloaded them later forcibly.
I wished mail clients too, had come up with such a feature that marks all the partly downloaded emails.