Sorry that this has been such a long time for a response.  If this can't help you, hopefully it will be able to help someone.

There is not property directly related to placing an image in the Label of a Field.  However, you do have the ability to retemplate it.  I have attached a sample that demonstrates how you can accomplish this.  Basically, all you have to do is grab the default LabelPresenterStyle from the DefaultStyles folder located at C:\Program Files\Infragistics\NetAdvantage 2010.3\WPF\DefaultStyles\DataPresenter.  In there, depending on if you are using themes or not, will depend on which file you look into.  For a default grid with no themes, look into DataPresenterGeneric_Express for the LabelPresenterStyle.  I added this style to my resources and then all I did was add another column definition to the layout grid of the Label and add an image in there.  It is pretty simple to do.

Then, all yoiu have to do is set the LabelPresenterStyle property of the grid field to that style and you are good to go. 

Hope this helps.