Hey Yapin. welcome to CB! There is a way you can stretch your image for your background.. When you're viewing the pic select the menu button and select zoom, zoom in to the size you'd like..then hit menu again and select "set as home screen image"...it should stretch to fit the whole screen
When I view the picture on my phone the 8130 I can press menu and the options are Full Screen, Fit to Screen, or Zoom, you can use any of these to get the picture the size you want it and then chose set as home screen...
Hopes this helps...