In the end, it was a power of 2 issue. Embedding the image into a larger image of 2048x2048 proportions made for the image to be visible on iOS. My confusion came from glancing at other image resolutions and not realising they were still power of two, but not in tandem. For example, 1024x512 is still valid in iOS but 1024x768 is not. 2048x1024, 256x512 etc are all usable.