how to change visibility of imagebutton at some answers

In my project I have used imagebutton for an attachment to the answers. But the problem is coming that when the expert has not any attached file then that imagebutton i.e. attachment symbol should not be seen at the run time.Please note that I have used Eval for fetching attachments from database.Please help me in code when the user does not makes any attachment to the answer. in short how to make the visibility of imagebutton false in the answers where there is no attachment made..