Add additional CSS class for particullar field in a node
You could try theme_field as well 


  Returns HTML for a field.


This is the default theme implementation to display the value of a field. Theme developers who are comfortable with overriding theme functions may do so in order to customize this markup. This function can be overridden with varying levels of specificity. For example, for a field named 'body' displayed on the 'article' content type, any of the following functions will override this default implementation. The first of these functions that exists is used:


THEMENAME_field_body_article()
THEMENAME_field__article()
THEMENAME_field__body()
THEMENAME_field()