How can the sensations of the meditation object (attention to the breath) be made more noticeable? While practicing sitting vipassana meditation, using the felt sensations of the abdomen rising and falling with each breath as the meditation object, there is the frequent flood of mental images arising and passing away. The mental images seem much stronger, i.e. more noticeable, than the felt sensations of the meditation object. Is there a mind or mindfulness technique to make the felt sensations more noticeable, thus improving concentration on the chosen meditation object?
Being mindful of the rise and fall of the abdomen is what you do when other phenomena are not occurring. If you are seeing mental images, note "seeing...seeing...." When the images fade, return to the rise and fall of the abdomen.

From Venerable Yuttadhammo's booklet, How to Meditate, A Beginner's Guide to Peace: 


  It is this rising and falling motion that we will use as our
  first object of meditation. Once we are able to observe the
  motion of the abdomen without difficulty, it will serve as a
  default object of meditation for us to return to at any time.


and:


  In regards to feelings, when a sensation arises in the
  body, one should fix one's attention on it, discarding the
  abdomen and focusing on the sensation. If a feeling of
  pain should arise, for example, one should take the pain
  itself as a meditation object.
  
  Any one of the four foundations may serve as a
  meditation object, as all four are aspects of reality. It isn't
  necessary to stay with the rising and falling of the
  abdomen at all times.


From Practical Vipassana Meditation Exercises by the Mahasi Sayadaw:


  In short, whatever thought or reflection occurs should be noted. If you imagine, note as 'imagining'. If you think, 'thinking'. If you plan, 'planning'. If you perceive, 'perceiving'. If you reflect, 'reflecting'. If you feel happy, 'happy'. If you feel bored, bored. If you feel glad, 'glad'. If you feel disheartened, 'disheartened'. Noting all these acts of consciousness is called cittānupassanā.
  
  Because we fail to note these acts of consciousness, we tend to identify them with a person or individual. We tend to think that it is 'I' Who is imagining, thinking, planning, knowing (or perceiving). We think that there is a person who from childhood onwards has been living and thinking. Actually, no such person exists. There are instead only these continuing and successive acts of consciousness. That is why we have to note these acts of consciousness and know them for what they are. That is why we have to note each and every act of consciousness as it arises. When so noted, it tends to disappear. We then go back to noting the rising and falling of the abdomen.


So just note "seeing...seeing..." with your mental images until they fade away then go back to the rising and falling.