Importing png images into Reporter??

I have found that using Keynote to make graphics to be used in Reporter is very quick and convenient. Un-filling the background and exporting as png provides a simple workflow. Confusion can arise when sharing the image to iPhone, sometimes it comes across as a png other times, for instance, using iCloud and a shared folder the image can arrive as a jpg loosing transparency on the way. My iPhone is short of memory but I do have the optimising images turned off. Does anyone know of a foolproof way of transferring pngs to iPhone and importing into Reporter. Much obliged to anyone who can offer clarity on what should be rather straight forward.