It's easy to add shapes to annotate a photo! Simply choose the shape and click-and-drag to add:
Choose the shape from the buttons on the bottom (it will highlight blue)
Push down the mouse button where you want the shape to start
Drag the mouse until the shape is sized how you want it
Release the mouse button
Editing existing shapes
You can also edit existing shapes by deselecting the bottom button by clicking it "off" then clicking on the shape to edit:
