This is your App!
Add the widget to your desk to get a picture of Adriana Lima different every 30 minutes. If you want to change the image before, then just click on it in the widget.
Also, if you want to see what photos the widget have, then open the application and slide the images scroll list.
On the subject of the battery don't worry. And remember that to add the widget you to your desktop, simply push and hold in a free space and select it from the list.
Depending on the device takes more or less space.