How can I check that I have not stretched my image?

Double click the image so both the black frame and blue frame appears. Zoom out a little so you have a lot of space around your image. This especially applies if your image is much larger than the page.

Next, hover over one of the tiny squares on the side of the blue frame until a black arrow appears.

Once you see the black arrow appear, hold down and push the blue framed image inwards or outwards until you see a red diagonal appears. This indicates that the image is 100% of it original size and not stretched.