Setting an movieclips image/background - actionscript-3

Question

I have a button and a small area which is a movieclip.
What i need is, on button press it inserts an image into the movieClip, it would have to be docked into the whole movieclip area essentially.
I have looked through multiple posts yet they have an overwhelming amount of information that I cannot figure out, this is so far what i have done :
B_Background1.addEventListener(MouseEvent.CLICK, setBackground1);
function setBackground1(event:MouseEvent):void{
var firstPic:MovieClip = new C_1BackgroundPIC();
addChildAt(firstPic, C_MainStage);
}
As I understand it, it adds an event to the button, then in the function it creates a new movieClip instance which has the Picture inside of it, and then adds it to the "MainStage" although using C_MainStage is invalid thus doesn't work, it does add a picture if i just use 0 as the position but it then adds it to the position 0, which i dont want...

Answer 1

It's a good start. When you use addChildAt, it expects a number as the second parameter. Presumably, C_MainStage is your MovieClip (which isn't a number). Presumably you want the current display index of C_MainStage (not C_MainStage itself). This can be had by using getChildIndex
Look at the following:
function setBackground1(event:MouseEvent):void{
var firstPic:MovieClip = new C_1BackgroundPIC();
addChildAt(firstPic, getChildIndex(C_MainStage));
//now, you may need to also resize your picture so it fits in the bounds of C_MainStage, if so, you can do something like this:
if(firstPic.width >= firstPic.height){
//if firstPic is wider than it is tall
//set it's width to the same as C_MainStage
firstPic.width = C_MainStage.width;
//now, to keep the aspect ratio (in case they don't match), make the scaleY the same value as the new scaleX
firstPic.scaleY = firstPic.scaleX
}else{
//do the opposite as above since the height is larger than the width
firstPic.height = C_MainStage.height;
firstPic.scaleX = firstPic.scaleY;
}
//now, you may want to center firstPic in the C_MainStage bounds
firstPic.x = C_MainStage.x + ((C_MainStage.width - firstPic.width) * .5);
firstPic.y = C_MainStage.y + ((C_MainStage.height - firstPic.height) * .5)
}