Hi micah,
Refer the below code. If you change the Height of the image you need to change the top property to adjust the text display in center.
<asp:FormView ID="userbigimage" runat="server" Width="100%">
<ItemTemplate>
<div style="background-position: center; overflow: hidden;">
<asp:Image ID="Image1" src="Images/Koala.jpg" runat="server" alt="User Image" ToolTip="userimage"
Width="100%" Height="170px" />
<div style="position: relative; top: -100px; left: 50%">
<asp:Label ID="Label1" Text="Sample Text" runat="server" ForeColor="Red" />
</div>
</div>
</ItemTemplate>
</asp:FormView>
Screenshot