SAmsung U600 wallpaper

[goodmorning]

i have my samsung U600. The phone is great but the problem is that i download a picture in the computer to that cell phone but when i make it as a wallpaper it is not stretched so it is just centered, it looks no good to be like that. But if it comes from "my photo' it will ask you to make it full screen,sumting lyk dat...but i wanna know how to make it a full screen that picture from downloaded ones..pls help me....thank you so much!