horatio's answer is good. I'd mark that the answer.
To offer some more information:
What density should be used to create mobile wallpaper?
The answer is "it doesn't matter". When you are printing images, you have complete control over the density. You get to decide how much information per inch of printing will be there by changing the DPI settings in your image creation software.
But with screen graphics, you don't have control over the density. The device does.
As such, any DPI setting you have in your image is simply ignored when presented on screen. All that matters is the overall pixel dimensions of your image.
A quick experiment to explain this:
- Open photoshop and create a 300 x 300px image at 72dpi.
- Save this as a JPG
- Now go to IMAGE SIZE and change the dpi to 300 (But do not resample).
- save this as a different JPG
Now open up both JPGs in your browser and you will notice that they appear exactly the same size: 300px x 300px.