Images visible in template, NOT visible once sent, visible to SOME email recipients, but NOT gmail recipients.

Images visible in template, NOT visible once sent, visible to SOME email recipients, but NOT gmail recipients.

Hi support, dear Sir or Madam,

Images sometimes visible, sometimes  not there.

When I send an email with pictures in it, the pictures are visible in my template and while I compose my letter, I change the addressee and add a few words.

The pictures are urls based on my partners dropbox account. The show up.

Then I send the mail.

Some recipients get the mail WITH all pictures visible.
A gmail account I sent to did NOT receive/see the pictures.
When I look in my Zoho sent folder the email contains NO PICTURES (just blank squares). 

I'm inserting the pictures as URLs using the insert image function of Zoho Mail. 

I've read quite a few articles here. *Nothing seems to cover this directly.

Please could you advise me how to solve this problem.

Thank you.