How to select the delta value for multi-valued field

Hi wonderful drupal people!

I have a multi-valued image field and I'd like to use its delta value with the Custom Formatters module.

However, I don't see the option available in the list.

Is there a way to do this? Otherwise I'd like to request for this feature to be implemented (or help on getting this done myself)

Thank you very much!
JieXiannn