Open the FLA file in Flash. For details, you can refer to Opening FLA File.
Now press "F11" to launch Library. To search needed image symbol easier, you can sort them by pressing "Kind" tab at the top of symbol list. You can see all bitmap symbols are grouped at the top of Library like this:
Bitmap Symbols in Library
Browse these symbols to find symbol you want to substitute. You can see symbol appearance at the top of library. To see properties of the necessary bitmap symbol, you can double click the symbol icon.
Bitmap Properties
You'll see image preview and options there. Check the image size. It looks like 101 x 68 pixels at 32 bits per pixel. To make your image fit original effects and animation, your image must be the same size as exchanged image.
You can resize or crop your image by using Adobe Photoshop. Open your image in it using File > Open... from the top menu. To resize or crop, you can choose Image>Image Size... or Image > Canvas Size... accordingly. Then save image using File > Save As.... For using in Flash, please choose one of the following image formats: JPEG, GIF or PNG.
After you prepare your image and save it, press Import button at the left and browse to locate your image file, click OK to close the window, your image will replace the original one.
At the bitmap symbols properties window we prepared the optimum quality for your Flash template. You can also change image quality to make it lower to reduce the published file size.
Now, if you finish your modification, it's time to publish your Flash movie. Here with detailed instructions and tips about it.