I agree with werner. First of all, 1024x768 is a lower resolution than native for your monitor, which doesn't look as good, but what's worse is the aspect ratio. You would be stretching a 4:3 image (1024x768) to 16:9 proportions. Everything will look stretched.
If you do need to change the resolution, all you have to do is right click on the desktop, and select "Screen Resolution" (Windows 7)
A suggestion for you, go here: http://interfacelift.com/wallpaper/downloads/
There are loads of free wallpapers that you can choose from, and you will be able to select the proper resolution before you download them. (1280x720)
Enjoy!