What he said. Using https instead of http would have been my suggestion. Also maybe the ‘Open Web View’ action as an option, but I have issues with that going full zoom without the option to pinch zoom, so have used “Open Link” instead, which opens the image externally with full control to zoom. Works fine for me on android, but from what I’ve read, ‘Open Link’ isn’t as friendly with IOS devices. Haven’t tested it, so I don’t know.
Just another thought for viewing external content, but I’ve recently started playing with the web view component to display PDF’s in app. I actually have it set to wrap the PDF url inside a google docs viewer url to view an external pdf inside google docs inside a web view inside the app. Works quite well actually.