RadListBox allows to declaratively bind the Text and Value properties of the ListItems, e.g:

<telerik:RadListBox runat="server" DataTextField="Text" DataValueField="Id">

Unfortunately, this is not possible for the ImageUrl property.

My Suggestion: please allow to also bind the ImageUrl declaratively as known from other controls, e.g:

<telerik:RadListBox runat="server" DataTextField="Text" DataValueField="Id"
    DataImageUrlField="..."

// and maybe even like this:

    DataImageUrlFormatString="..." DataImageUrlFields="...">