I prepared a sample project which covers the scenario you describe:
There is a repeater control, which has a user control in its item template. In this user control there is another user control and an image button inside it, which is the target element. Another user control is loaded in the OnAjaxUpdate event of RadToolTipManager and some value is passed to it.
Please, examine my test project which is attached. I hope that it is helpful.
As to my assumption about calling the user control's DataBind() method, I assumed that you could have only an image, but its url is evaluated and taken from the DB as shown in our online example. In this case, when you have code blocks like <%%> in order to evaluate the expression you should call the image's DataBind method.
All the best,
the Telerik team