Set the size – instagram use 1000px as the width of their images, so that’s a guide, 600px is usually fine.
If you make sure “constrain proportions” is checked your images will keep their aspect ratio, and not look weird.
If you have already added an image to can resize it by right clicking on the image (or CMD click if you don’t have a right click sorted) and dragging the little squares.
Who does this?
People who want to have a bit of control over the layout of My Studies pages, like Phil Tyler.