How can I put individual Graphics in Datagrid Cells?

I want to put little arrows inside a DataGrid to mimic the kind of display shown in this Excel file:



But the XAML code defines individual columns instead of individual cells:
Code:
                    <data:DataGridTemplateColumn Header="Prev. Mo. Trend" IsReadOnly="True">
                        <data:DataGridTemplateColumn.CellTemplate>
                            <DataTemplate>
                                <StackPanel>
                                    <Image x:Name="img1" Width="11"  Height="12"  VerticalAlignment="Center" HorizontalAlignment="Left"
                                   Source="/MedAnalyzer;component/Assets/greenuparrow.png" Visibility="Visible"/>
                                    <TextBlock  VerticalAlignment="Center" HorizontalAlignment="Right"  x:Name="Prev_Mo_Trend" Text="{Binding Prev_Mo_Trend}">
                                    </TextBlock>
                                </StackPanel>
                            </DataTemplate>
                        </data:DataGridTemplateColumn.CellTemplate>
                    </data:DataGridTemplateColumn>
I can add text into the Datagrid by data binding with an observable collection. But this is text data. What about replacing gaphics? Do I need something extra special?

This is the results of my efforts



I do not need to have the graphic and the text share the same sell. That part can be overlooked. How can I just replace the arrow with a different error within the cell?