1. Hover over the image and click on the pencil icon that appears at the top-right corner.
  2. In a lightbox that appears, you can edit the figure with multiple tools:
    • Pencil: free drawing lines
    • Geometric shapes (e.g. rectangle, circle)
    • Lines: straight lines
    • Arrows or arrowheads
    • Asterisks
    • Texts
  3. You can also change the size, color and opacity of the marks or texts.After editing the figure, you can confirm the changes by clicking the save icon to the right end of the toolbar, or discard them by the X icon next to it. 

Please note: annotations are saved in a layer separate from the image.