In deep trance / Samadhi meditation does contemplating on the subject, before hand, give rise the mental image or Nimitta, or does it arise while contemplating? Put it in another way, do you have to stop the contemplation for the Nimitta to arise or does it arise while contemplating?
The mental image (Nimitta) as per some teachers appear as a bright radiant light. You have to bring your attention to a particular stimuli and retain the attention on the chosen stimuli by continuously bringing your mind to the chosen stimuli. At some point you might get the Nimitta. This does not happen for everyone and every time. You should do active thinking about the subject of concetration followed by letting go of it altogether to progress futher before the Nimitta apears.
If you are doing the above as Vipassana you have to look at sensation (or phenomena that is know and felt) and continuously keeping your mind on it. Then you will see the arising and passing of sensations, which perhaps you can consider the Nimitta.
Also see: Nimitta: The radiant and blissful sign [How to enjoy and benefit from your meditation] and Nimitta and Anuvyañjana: Signs and Details [Sense-restraint & wise attention: how to master the senses] by Piya Tan