Following snippet code shows how to add background image in WebDesktopManager.

<ISWebDesktop:WebDesktopManager ID="WebDesktopManager1" runat="server" Height="100%"
    Width="100%" ShortcutIconsVisible="True" EnableContextMenu="true">
    <ImagesSettings OverlayBackground="../images/aqua_images/aqua_bg.jpg" />
    ...
</ISWebDesktop:WebDesktopManager>

By default, the background image will rendered as img element such as shown below:

<img style="width: 100%; height: 100%; position: absolute;" src="../images/aqua_images/aqua_bg.jpg">

Using DOM Explorer, this element is the firstChild of WebDesktopManager. We can use OnInitialize client-side event to manipulate this element. For example: removes its height and width style to prevent the background image stretched based on the screen resolution. Thus the size of the image will be as-is.

function WebDesktopManager1_OnInitialize(controlId)
{
    var WebDesktopManager1 = ISGetObject(controlId);

    WebDesktopManager1.GetFrameElement().firstChild.style.height = "";
    WebDesktopManager1.GetFrameElement().firstChild.style.width = "";
            
    return true;
}

Hope this helps.