Signatures in Mail App: Images show up in mac mail client, but disappear (null) in Iphone mail app

There seem to be a variety of strange issues with using signatures (with images) on the Iphone. Here's ours:

We added our company logo as an image to the signature in our Iphone (settings>mail>signatures). Initially, when sending emails from a phone, the images showed up in the mail app for both phones and macs.

The next day, we noticed that when sending emails from/to the iphones, the images were missing on Iphones, but still present in mac mail (same piece of mail).

This is a strange and vexing problem. What shows up on Iphones (day 2) is a little box (that is the size of our logo) with "null" in it. But on mac mail the image is fine.

This is only on emails sent from one's Iphone. Any assistance or advice would be deeply appreciated.