Maybe I should just know, but what image viewer is that? Gwenview?
What happens if you open a jpeg in your browser, or a website that includes jpegs? What about if you open a jpeg in GIMP? What if you open a JPEG in GIMP and convert it to a PNG? And when you convert it back?
Is this only limited to JPEG format images? I assume yes, since you managed to screenshot that .PNG for us? Can you see this JFIF?