Depending on the nature of the Item Source for your grid, you can change the image source by binding it to a property / column value that changes according to the value you are interested in:
<DataTemplate x:Name="trFalse" >
<Image Source="{Binding ImageSource}" Height="20" />
</DataTemplate>
Where ImageSource is the property holding the image source according to the "other" cell value.
You can also use a value converter if you have no control over the object properties.