To change desktop wallpaper, right click on a clear side of the desktop, this will bring up a dialog box. Then select properties from there select the desktop tab there is a list of image files from which to choose.
You can select one of those (a single click will
set it in the preview monitor to let you see how it will be displayed) or you can browse if you browse you will be given access to "windows" where you can select virtually any image file on your computer. You will notice a drop-down menu.
The menu has 3 selections if you selelct "stretch", Windows will stretch the image to fit the decktop Low resolution images may not look very good If stretched. If you selecct tile, the image will be repeated as many times as nessary to cover the desktop. If you select " center" the image will be shown in actual size and centered on the desktop.