What is the default resolution of a wallpaper that fits the homescreen exactly in an Android phone? On an Android phone the wallpaper spans across all of the home screens.

So what size should pictures be to fit properly as custom wallpapers?
Here are the correct wallpaper dimensions for the phones cited in Leimi's answer:


480x320 for devices like the HTC Wildfire 
640x480 for devices like the HTC Hero or Legend 
960x800 for devices like the Nexus One 
960x854 for devices like the Motorola Droid 


If you device isn't listed there is an easy way to figure out the dimensions of the wallpaper based on your particular phones resolution:


2x the width and 1x the height


so for example my Moto Droid has a screen resolution of 480x854 so I want my wallpaper to be 2x the width (480 pixels) and 1x the height (854 pixels) = 960x854.

Here is a chart with most devices specs to help find the resolution.

Also check out this Lifehacker article that explains Android wallpaper sizing well.