Hi, on my 11.1 version for WebPanel I used stylesetname property where I have background image set correctly:
<igmisc:WebPanel ID="ViewPanel" runat="server" CssClass="viewPanel" StyleSetName="Header"Width="805px" Height="870px">
<ClientSideEvents ExpandedStateChanging="CancelXpand" />
When I downloaded 15.1 version and replace with WebExplorerBar background image disappeared.I used the same exact path:
<ig:WebExplorerBar ID="WebExplorerBar1" runat="server" CssClass="viewPanel" Width="805px" Height="870px" GroupExpandBehavior="SingleExpanded" StyleSetName="Header" >
Please see attached two screen shoots:
First Image using WebPanel
Second Image using WebExplorerBar
Thanks so much.
Modify the following code and see if it will work.
<!DOCTYPE html><html><head><style>body { background-image: url("paper.gif"); background-color: #cccccc;}</style></head><body><h1>Hello World!</h1></body></html>