The server tag is not well formed

You missed double quotes for itemname1 in onmouseover.

Try:

<asp:Image ID="Img1" runat="server" style="cursor:pointer;" ImageUrl='<%# Eval("photoLink1") %>' onmouseover="Tooltip(true,'<%# Eval("itemname1") %>',event);" />

Few discussion that might help:
ASP.NET ItemTemplates, EVAL() and embedding dynamic values into controls[^]
Image Url Eval - Similar discussion[^]

Attribute ImageUrl must be in quotation marks "", as all other XML attributes.

—SA

as per my experiance such things could be done this way...
1) it get you rid of lots of escape sequencing stuff..
2) gives you better flexibility on what you doing
3) more redable and managable by other programmers
4) you can check for nulls etc..

I personally always do such things in a function on server side and then call it in aspx page like

<br />
onmouseover = <%# GetToolTipFunction(Eval("Iitemname")) %><br />

in my code behind

<br />
  protected string GetToolTipFunction(object itemname)<br />
  {<br />
      // do any other stuff, like checking nulls etc...<br />
      String functionStr = "Tooltip(true,"+ itemname.ToString() + ",event)";<br />
      return functionStr;<br />
  }<br />

it may look like 2 lines of more code, but trust me it will make your life much easier in long run.