Outlining a technique developed to create irregular mask shapes in Pixelmator.
Locate an appropriate image. In this case I’m using a graphic of a bunch of avocados.
Activate the Ellipse tool under the Shape category from the tool box.
Set the Fill to Color, Choose Red and set the stroke to None.
Zoom in and draw the ellipse on a small section of the graphic.
Right–click on the Ellipse and Choose Make Editable from the Context Sensitive Menu. At this point you’ll be able to move the individual four points and manipulate the curve editing arms.
Adjust the placement of the points by dragging then into place and adjust the curves by pressing the command key and dragging the ends of the control arms. Set the Layer blend mode to multiply to see through the shape as you edit.
Control the shape of the curve between the points by clicking on the edit arms and dragging. Click on an edit arm and drag to adjust both sides of the curve symmetrically. Command click to adjust one section of a curve.
Continue working your way around the image by right clicking on line and choosing Add Point. Adjust placement of points and angle of curves as you go. Hint: Try to keep arms in line with the uncovered portions of the graphic. This will prevent unexpected results.
Once you are satisfied with the shape, target the Shape Layer and Command click on it, loading it as a selection.
Hide the Shape Layer and target the Background Layer. Right click on the background Layer and choose Add Mask. The image is ready to be composted over a different background.