Put some alt text in iamges of lines for asp:TreeView

I am using following as my tree view
<asp:TreeView ID="tvCategories" runat="server" ShowLines="true" ShowExpandCollapse="true" NodeWrap="true" ClientIDMode="Static" />
it is generating images for them like 
<img alt="" src="/WebResource.axd?d=dfVL09bUhBneFGh9D0vDj08RF3Nrn9hb0Lh74Ri0hqgqmkAuOeYNlcDnBs2UP4NuOmbnPWYc2_OLskPjZI7ZHoMPKIYGQVmcGAiYgA2&amp;t=635065079764912806"> what i simply want's is to inject some text in alt like alt="Categories"