Set the system (home screen) wallpaper from an SD card image file.
JPG and PNG are supported.
If no image is specified, the default system image is set.
Scale: if the image is smaller in width or height than the launcher-specified values, scale it up to those values while keeping the aspect ratio.
Crop: if the image dimensions (possibly after scaling) are greater than the launcher-specified values crop it down to those dimensions. This can be necessary to prevent Android-internal problems.
Note that it can take more than 20 seconds for this action to take effect.