STEPS TO FOLLOW

Inserting Layers and Naming them

• By default you will have a layer in your timeline window. Insert one more layer, totally you need two layers to mask an object.
• Rename the top layer to "Mask" and the layer below that to "background".

Creating Shape Tween

• Import your picture to the "background" layer.
• Using Oval tool from your tool box, draw a circle in your "Mask" layer and delete it's border.
• Drag the circle to one end of your picture.
• Now go to"frame 40" of your "Mask" layer and press "F6" to insert a new keyframe.
• Now go to "frame 40" of your "background" layer and press "F5" to insert frames, so that your background image is available all through your mask.
• Select "frame 40" of your "Mask" layer, that is your new keyframe, Keeping the playhead on "frame 40" of "Mask" layer, drag the circle to other end of your picture.
• Now go back to "frame 1" of your "Mask" layer, keeping the playhead on "frame 1" of your "Mask" layer, select Shape tween in your properties window.
  

Masking

• Right click on the "Mask" layer (the area where you named the layer not where the frames exist) and select Mask.

Your Mask is all ready. Press Ctrl+Enter to view your Mask.