Images in emails not downloading

Can any of you clever people out there suggest why this could be…

I have 2 very similar scenarios in 2 apps whereby I insert images into an email but in one case the image always comes out perfectly in the email and in the other the recipient needs to download the image and I cannot see why.

I have tried hosting the images from different sites and any other variables I can think of but no luck. They are both coming to my same inbox so I can’t see how it could be my email settings.

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Am I missing something obvious / has anyone else seen this behaviour?