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.
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.
The mental images seem much stronger, i.e. more noticeable, than the felt sensations of the meditation object.
If it's hard to direct the mind to attend to something, if it's easily seduced and distracted by anything other than the object you want to give attention to, this means samadhi is lacking. This is what gives one the ability to develop a unified mind (one pointedness), resting it on an observed object without distractions; "applied and sustained thought". So, it might come in hand to train samatha as well.
Is there a mind or mindfulness technique to make the feeling sensations more noticeable, thus improving concentration on the chosen meditation object?
Yes there is! Being mindful of whatever arises, i.e. mental images, clearly noting their arising and ceasing, will strengthen your mindfulness and concentration.
The point of vipassana is to come to see the impermanent, unsatisfying, and uncontrollable nature of all phenomenon, and here, my friend, you have it! The goal is not to maintain concentration on one object of your choosing.
edit: I'm assuming you're practicing vipassana? If this is the case, please edit and add the vipassana tag, as that would change the appropriate answer to the question
Mental factors has Sensations associated with them. So any metal activity will have a Sensation. You have Perceived association with each Sensation. So one Sensation brings up associated memories or fantasies, which create more Sensation which continuously bring up more such thoughts. This is how Metal Proliferation happens which is a Verbal Fabrication. To remove the Verbal Fabrication you have to cut out Thinking and Pondering by with Sustained and Applied Effort to Direct Your Mind to the meditation object with the Intention, Continuous Reviewing to ensure your focus is on the object. (Also see my answer https://buddhism.stackexchange.com/a/12342/295 for further details.)