Galaxy S6 :: How To Open PNG Files From All Phone

Ok, to try and clarify...I have had all of the Samsung Galaxy S phones except the S5. I've moved all my photos & screen shots from them to my pc over time, generally via Kies or usb transfer. I've noticed that I have now got a huge number of .png files dated from about 2012 onwards that I can't open. Previous png files can be opened-they appear to be screenshots or the like. So I don't know what has happened. They can't be opened using any of the options on my pc. (I recently changed to Windows 10 from 8, but it was doing this prior to that) I can't re save them because they can't be opened, nor does changing the file name allow them to be opened.

I've also noticed that my current S6 makes all my screenshots .png files as well. The only way around this at the moment is to upload them to Google drive and change it to a jpeg file at the time. I have to do this for each shot one at a time. I'm assuming I can then move these across from Google drive as jpeg.

So, I need to know how to change the default on my S6 from .png to .jpeg for screen shots. How to change my previous .png files back to jpeg so I can open them! I have copied all of them to a usb stick and tried to open them on my work computer, but that also won't open them under any program. I realise that isn't an Android issue in itself, but as all of these images have come from my Galaxy S phones originally, I don't want to delete them without being able to see them as I might want to keep them...but without knowing what they are...