Unfortunately, the resource key doesn't work quite like that.  You will have to fill the AlternateText and Tooltip properties manually.  If this is inside of a skin, the easiest way is to use databinding syntax. 

<asp:Label ID="lblWorkerName" runat="server" Text="First name:" resourceKey="FirstName" ToolTip='<%#Localization.GetString("FirstName.ToolTip")%>'/>
<asp:Image ID="imgTag1" runat="server" ImageUrl="~/images/img_not_avail_300X450.png" resourceKey="ProductImage" AlternateText="'<%#Localization.GetString("ProductImage.AlternateText")%>'" ToolTip="'<%#Localization.GetString("ProductImage.ToolTip")%>'" />

If this is in a module that you are building, it's probably easier to just set these properties from the code behind file.

Sorry for the premature approval.  I was looking at the wrong control when I answered.

I chose to not use the code behind method as it just didn't seem like the right option for the amound of labels, images, etc I have on the page.  So I chose to add them using the databinding syntax like this:

<

asp:Label ID="lblSite" runat="server" Text="Site:" resourceKey="Site" ToolTip='<%# Localization.GetString("Site.ToolTip") %>' ForeColor="White" Font-Bold="true"></asp:Label>

Problem is now they don't display at all.  I have the reference in my code behind to DotNetNuke.Services.Localization and added the imports statement to the .ascx file like this:

Still no success.  I have tried adding Localization.GetString("Site.ToolTip", Me.LocalResourceFile) to the control and still no success.  What else am I missing?