About Screen shot of Mine craft

When I take a screenshot in-game, black bars (or triangle-like black shadows) appear on the top half and the bottom-left corner of the image. These black areas cover parts of the game that should normally be visible. This happens every time I save a screenshot, and all images are affected in the same way. Removing the texture pack did not fix the issue. This problem only occurs with in-game screenshots. I would like to show an image, but I’m not sure how to do so, so I tried to explain it in words. I hope this makes sense.