| First of all, the colour panel is not where you want to change the alpha in this case; it's only for changing the colour of what you're about to draw, or of a raw graphic currently selected on stage (like if you'd drawn a blue box, selected it, then you could change its alpha with the colour panel).
What you want is to create a simple movieclip (call it image_basic) with one frame only, and have it contain the image in question. Once that's done, do this:
1) Create a new movieclip called image_tween; give it 20 frames, and 2 layers.
2) On the top layer, create a new keyframe on the last frame; using the Actions panel, insert a stop() action here.
3) On the second layer, first frame, drag image_basic from the library onto the stage.
4) On the last frame (same layer) make a new keyframe (not a blank one, though; you want the image still there).
5) Back on frame 1, select the image_basic. In the Properties panel, you should see it mention "Instance of: image_basic". If you don't you haven't selected it properly. In its Color dropdown, select alpha and set it to 0.
6) Now click on the current frame on the timeline (make sure it's the layer with the images on it). Using the Properties panel again, create your Motion Tween.
7) Double-click your button symbol in the library to edit it. Drag an instance of image_tween onto the button's second frame.
That should do it. |