Symbols are abstract elements in Flash. The best way to describe them would be to think of them as containers. When you convert an image to a Symbol, you aren't actually converting it as much as you are placing it inside a Symbol container.

Why do you need to do this? Well, Flash doesn't really have the ability to do much with objects that are on the Stage. But, if you place an object inside of a Symbol, Flash can manipulate properties of that Symbol. For example, it can move it around the Stage, stretch it out, or change its transparency — which is the property we are going to change to make the images fade.

Grasping the concept of Symbols is not easy. It is best to work with them for a little bit, and you will quickly understand the way Symbols work. Just remember: 

When you should convert an object to a Symbol:
When you want to animate, stretch, fade, position, move, make clickable or interact with an object on your Stage.

When you do not need to convert an object to a Symbol:
When the object is static and will not change at all, like a background image or a logo.

Converting your Images to Symbols

Converting an image to a symbol is generally very easy to do, however in our case it is a little trickier since we want each of our four images to be converted to their own symbol. We will have to first hide some of the images to make sure we only convert one image at a time into a symbol.

Click the small dots under the eye icon to hide the top three layers.

You will see the images on those layers disappear.

Next, and this is important, click once on the image that is showing on your stage to select it.

Under the Modify menu, select Convert To Symbol.

If the option is grayed out, it means you haven't selected the image on the Stage. Click once on the image that is on the Stage first.

Type the name of the image in the symbol dialogue box, and choose Graphic for the type option.

The registration won't really matter in this tutorial, so you can leave the upper-left box checked, and for people using CS4, the Folder option can remain Library Root.

You will know you were successful in converting the image to a symbol if you see a small circle appear in the center of the image. The border may be a different color depending on the Flash version you are using.

Now hide the layer with the image you just converted to a symbol and reveal the next layer up.

Repeat the process of converting the image to a graphic symbol, naming it appropriately. Do this will all four images so that they are all symbols.