There is no image when the X server starts, unless you mean the nVidia logo.
There may be an image when your window manager starts, or when you use a display manager there may be an image from there.
First you need to find out what display manager or window manager you are running (a display manager displays a loginbox and is called something ending in "dm", like xdm or kdm). The window manager displays your icons, taskbar, popmenus etc.
When you know what program is displaying it, it could be possible to change it.