Corrupt images display to users when not logged in to WP - using Upfront

After editing and checking my site whilst logged in as admin, I discovered that when not logged in to WP images are being displayed differently. All images contained in tabs within pages are showing with a choice box for setting links.
I thought it may have been a cached copy of the pages so I tried installing WP Super Cache and then uninstalling it again but this made no difference. I have deactivated Hummingbird and Defender, neither of which made any difference. The corrupt images display on Chrome, Android, IE browsers equally and different display sizes don't effect the result.
I suspected caching at the server so I checked a development version of the website that is hosted by a different company and received the same result so now I am thinking it may be an issue with the Fixer theme.

Any assistance would be greatly appreciated.
Thanks