George,
You have to use high-quality images for your desktop. The size of the
image needs to be the same size or larger as your computer display
resolution. Most run at 1024X760 pixels, you will need a picture at
least that size.
If you attempt to use a smaller picture, you will need to center it or
tile it. If you stretch it, the computer will stretch the picture from
it's small size to a large size, and the effect is blurriness or
pixelation.
I would do a good search for free desktop images, and then download the
actual images, and not the small thumbnails that Google Images
provides. You usually have to get this from the originating site, and
you usually have to right click the image you want and select "Save
image as" to get it saved on your hard drive.
Hope this helps.