You can do something like this:
ASPX:
<ItemTemplate>
<asp:Image ID="imgBooleanState" runat="server" AlternateText="State" ImageUrl='<%# GetBooleanState(Eval("MyBooleanValue")) %>' />
</ItemTemplate>
Code behind:
protected string GetBooleanState(object state)
{
bool result = false;
Boolean.TryParse(state.ToString(), out result);
if (result)
{
return ResolveUrl("~/path/tick.png");
}
else
{
return ResolveUrl("~/path/cross.png");
}
}