You can't change a local value in style because of Value Precedence. This should work.
<Image>
<Image.Style>
<Style TargetType="Image">
<Setter Property="Source" Value="{StaticResource IconSave16}" />
<Style.Triggers>
<DataTrigger Binding="{Binding Path=IsEnabled, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Button}}}" Value="False">
<Setter Property="Source" Value="{StaticResource IconSaveInactive16}" />
</DataTrigger>
</Style.Triggers>
</Style>
</Image.Style>
</Image>