In this thoroughly explained, detailed lesson, I will show you how to create a really special picture effect using a mask technique. You don't have to use action script code to make this lesson. Using this lesson, you will also learn how to use Selection Tool for cutting some picture elements, how to create Motion Tween, how to create mask layer and much more. You can use this effect for some flash header, banner, for some presentation...
Example: Step 1Create a new Flash Documet, choose File > Import > Import to Stage (Ctrl+R) and import any picture. Frame rate set to 32 Fps (Frames per second).

Step 2Double click on layer 1 to rename its name into pictures.
Step 3
Insert a new layer and name it mask.
Step 4Lock the "pictures" layer, select the mask layer, take the Rectangle Tool (R), Stroke color must be switched off, under fill color choose any color and draw a "rectangle" like it is shown on the picture below!

Step 5Take the Selection Tool (V), and do like it is shown on the picture below!
Step 6Finally, your "rectangle" must look this:
Step 7Select the "rectangle" and press F8 (Convert to Symbol). In Convert to Symbol window under Type choose a Graphic, name it whatever you like and press ok. Look at the picture below!
Step 8Click on the twentieth frame of layer pictures and press F5. After that click on the twentieth frame of layer mask and press F6 (keyframe). While you're still on the twentieth frame of layer mask, using arrows key, or by mouse, move your "rectangle" on the position like it is shown on the picture below!


Step 9Click on the first frame of layer mask, open the Properties Panel (Ctrl+F3) and under Tween choose Motion.

Step 10Select the mask layer and choose right-click and select mask to turn the layer into a mask.

Step 11Click on the twentieth frame of layer mask, open the Action Script Panel (F9) and paste this script:
stop ();
Now, we're done for the first picture. You can apply this effect on many pictures if you like.
Enjoy!
Download source file (.fla)