If you long press the Android luncher and then you pressed Wallpapers an app/activity is opened and you could select the wallpaper (it have a gallery view at the bottom). Where I could find the source code for this app/activity in the Android source code (can you point me to the file location or the github url for the app/activity specific path).
The wallpaper chooser for images should be the following part of the launcher
/packages/apps/Launcher2/../WallpaperChooserDialogFragment.java
There are also live wallpaper related packages, for example this could be the chooser for live wallpapers.