The iPad2 screen resolution at this time is 1024 x 768
iPhone 4 screen resolution is 960 x 640
iPhone - iPhone3 screen resolution is 480 x 320
the max size of image for iPhone is 1019x1019 if use it over the size it will not show on the screen while playing
I am not sure for the iPad but...
You can use the big image as the background but it will make your game lack on the device.
So I suggest you need tile background or separate background to many small parts would be better than use only one big image.