You need to create an application skin then add a bitmap to it, this is the simplest as below, just right click on your designview stage select create skin then add the following line at the end of the new skin file just before the contentgroup
line to add --->
<s:BitmapImage id="bgImage" top="0" left="0" bottom="0" right="0" fillMode="scale" source="@Embed('mybackground.jpg')"/>
place above this line --->
<s:Group id="contentGroup" width="100%" height="100%" minWidth="0" minHeight="0" />
you can play around with the alignment and fillmode to get the effect you want
but it did not show image
1. did you create a new application skin ?, and was the skinclass called in the application declaration
2. did you put an image in the project source folder to use
3. did you correctly name the image in the skin file.
how to set back ground image without using bitmap tag help me.
You really do need to change the skin, if you want it to be more dynamic you can use an image var in your main app that the skin image binds to(in this case you would use mx image rather than the spark bitmap image as bitmapimage doesn't directly support image binding.
I have created a simple example for you so if you are having trouble with creating the skin you can simply use the one in the example and change the embedded image to the one you want to use.
http://gumbo.flashhub.net/appskin/ - source included