Basically, an image will be placed inside a Symbol, it may be a Graphics or a Movie Clip. Ok, here is the sample...
Now I am going to exhange an existing image for the image shown below on the flash movie.

1.

First you will have to find wheather the image layer is locked. If it is locked you will have to unlock it. To unlock a layer, you will find something like a padlock,

2.

What you have to do is, just click on the padlock, which have locked your image. Then the layer will be unlocked, as shown below.



If it is unlocked in default, then no worries....forget it....

3.

Now double click on the image, what you can see in the movie.

4.

And double click again and again until you get the last symbol,




Earlier it was shown as Scene 1, when double click on it for the first time, it has gone inside the Movie Clip named " mc_img2", and again when you double click, it has gone inside the Graphiics Symbol named "gfx_img-1" as shown in the image. Thats the last symbol where this image is placed at. How do you know is after that how many times you double click on th image, even it still in the same symbol.

5.

Now you will have to create a new Layer, to create a new layer click on the "add new layer" button as shown below on th image.

6.

Ok...Now this is the time to bring your new image, which gonna replace the old image, but make sure that you are in the new layer.. Ok go to File - Import





then browse the image you want to exchange.

7.

Then you will see on the First Layer your new image will be placed and on the second layer your old image will be placed,

8.

Then just delete the layer which includes your old image

It is Done..now you have replaced your new image with the old image