How to put a picture within an application as the ICON in android?

In each application there is a xml called in AndroidManifest.xml

AndroidManifest.xml inside look at the line, as follows,
<application android:icon="@drawable/icon">
default value is the icon, that icon.png save png's said.
There are two ways to change
(1)
Replace your image drawable directory icon.png
(2), or to put in lower case name xxx.png drawable directory
And change the information under AndroidManifest.xml, set your image file name
<application android:icon="@drawable/xxx">
The second way is recommended.