Here's my GridView HTML:

<asp:GridView ID="gvPortfolioImages" runat="server" AutoGenerateColumns="False" DataSourceID="ldsPortfolioImages">
    <Columns>          
        <asp:TemplateField HeaderText="Image" SortExpression="Filename">
            <ItemTemplate>
                <img src='<%# Portfolio.GetImageURL(Eval("Thumbnail").ToString()) %>' alt='<%# Eval("Thumbnail") %>' />
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>

I'm get the following error where I'm trying to call Portfolio.GetImageURL():

The name 'Portfolio' does not exist in the current context

I've seen functions called like this before, but it doesn't seem to be working for me. Can anyone tell me what the problem is?