To modify the proportions of a shape, click, hold, and drag one of the yellow squares that appear on the shape. To rotate the image/shape, click and drag on the circle at the top. Hold down the Shift key before you start dragging the corner to keep the image proportionate. Note that depending on the file type, your image may become distorted if you make it bigger. To resize it, click and drag one of the little squares that appear in the corners. To move it, click on it, hold, and then drag it to the desired location.
The first three options will work with photos/images as well as shapes.
