Draw along the outside edge of the image.
Remember that you are actually painting on the mask. As you paint with black on the mask, you are hiding that portion of the image. (If you see black brush strokes, you are painting on the image, not the mask. Undo your brush strokes and then click the mask thumbnail on the Layers panel.) If you hide too much of the image, you can switch the stroke color to white and paint over the hidden area to reveal the image.
When you paint on the mask, you are altering the visibility of the masked image. Painting with white makes the masked image completely visible. Painting with black makes the masked area invisible. Painting on the mask with shades of gray makes the masked area partially visible. The lighter the shade of gray, the more you see of the image; the darker the shade, the less you see of the image.
Tip
You can press X to quickly swap the stroke and fill colors as you paint on the mask. This is the shortcut key for Swap Stroke and Fill colors on the Tools panel.