Try something like 

<dxe:ASPxButton ID="btExpandAll" runat="server" EnableTheming="False" EnableDefaultAppearance="False" AutoPostBack="False" EnableViewState="False" Cursor="hand"
  GroupName='<%# Container.Column.Caption %>' ToolTip="Expand All"
  Image-UrlChecked='<%# string.Format("~/App_Themes/{0}/GridView/gvCollapsedButton.png" , this.Theme) %>'
  Image-Url='<%# string.Format("~/App_Themes/{0}/GridView/gvExpandedButton.png" , this.Theme) %>'
  Image-AlternateText="Expand/Collapse" >
  <ClientSideEvents Click="btExpandAllClick" />
  </dxe:ASPxButton>