Finally, I found how to do it :
<xcdg:Column FieldName="IsModified" Width="20" ReadOnly="True">
<xcdg:Column FieldName="IsModified" Width="20" ReadOnly="True">
<xcdg:Column.TitleTemplate>
<DataTemplate>
<Image Source="/Resources/Modif.png"/>
</DataTemplate>
</xcdg:Column.TitleTemplate>