Paste a screenshot into the window with ⌘+v and then hold a key and drag to mouse to draw shapes.

• A and drag for arrows
• L and drag for lines
• B or R and drag for boxes (rectangles)

Pressing S will select the most recently-placed item, after which ...

• Arrow keys will move the item
• Holding Shift and dragging the mouse will move it
• Holding Option or Alt and dragging the mouse will resize it
• Backspace will delete the item

Pressing X will export the image back into your clipboard.