Hi Neda,
Sorry for your inconvenience, Answerers are there to help you out in any technical problems
However for your current problem,
set you Visible property in Image Tag only
Visible='<%# string.IsNullOrEmpty(Eval("nameOfImageField"))%>'
And it will make visible only those image for what you have image Url avalilable
Thanks and Regards,
Rk_Hirpara