Open Startmenu->Run type regedit and press ok to open registry editor.(shows a tree like structure of directories at left)
In that hierarchical structure in left, navigate to registry entry
In right side pane see a number of values placed in a table format. choose the value named Wallpaper from there and double click it. Now you see a box with value name as Wallpaper and value data as Path to the image file .There give the full path of the image(bmp file) which you want to set as logon wallpaper by deleting previous path and writing path to your bmp file e.g. C:WINDOWSlon.BMP (to set image lon.BMP file as log on wall paper).
Also double click on WallpaperStyle and change it’s value to 2 to get a stretched wallpaper at logon.
Put that bmp file in windows directory for better results.
If that is a jpeg file, convert to bmp file by opening in Windows Image Viewer and save as bmp.