The problem is because the path cannot be resolved. I suggest replacing regular images with asp:image and a relative path if you are using the same master page for content from different folders .The reason you are having problems with the background color is because of the same reason .The style sheet link path does not get resolved. I would strongly suggest taking a look at themes
http://msdn2.microsoft.com/en-us/library/ykzx33wh.aspx
A quick and dirty way .. I do not recommend it is to add the css link via code behind
Literal cssLink = new Literal();
cssLink.Text = string.Format("<link href='{0}' runat='server' rel='stylesheet' type='text/css' />", ResolveClientUrl("~/myfolder/stylesheet.css"));
this.Header.Controls.Add(cssLink);
Jeev
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