There are a variety of formats supported by PNG, my version of the PNG suite contains 157 different files. The File-based VIs will handle 154 of the 157 files. The string-based one will handle 59 of the 157. Clearly NI has written/licensed a nearly compliant PNG reader, why it is not using the same one in both cases is beyond me.
I was annoyed when I first saw the string based VIs hidden deep in the palettes (having written my own). After some quick testing the crappiness (technical term) was soon apparent so I was glad to have made the effort. The simple .NET solution may be an option for you if you are on a Windoze machine.