[Mark Dowd]"How do I add a custom image mask to a video clip?"
If you are simply wanting to mask out an object or area in a single video clip, load the clip into a group. Click on the Mask Tool (Option B for the bezier mask tool) and begin to mask out the object. Once you complete the mask, you will have a tab called Mask in the Inspector that you use to make adjustments to the mask.
You can also create a shape, whether using the bezier tool or the shapes in the library and use that as a mask by making it an image mask. Place your clip in a group and then create a new group and place your shape in Group 2. Highlight your clip in Group 1 and control-click or right mouse click and select image mask. In your inspector you will have a well for the shape in Group 2. Click on the shape in Group 2 (not the Group) and drag it to the well in the Inspector. Turn off Group 2.