The highest ideal of Namasmarana might well be the popular phrase from the scriptures :
Sarvadaa Sarvakaleshu Sarvatra Harichintanam
The Mind's involvement :
In the Gita, Arjuna asks Krishna, about the wavering of the mind. Krishna says that repeated practice (Abhyasa) and dispassion (Vairagya) are the tools by which to conquer it.
In the beginning, Namasmarana may be mechanical, but in due course, mind is reined in. Recognizing this difficulty, the Sandilya Upanishad, mentions three levels of Japa, Vaikhari (loud), Upamsu (whisper) and Manasa (mental).
Significance of the form : Focus on a single name and a single form is recommended for gradually increasing the levels of contemplation. The gross appearance of the picture itself is not referred to as significant, it could be anything, don't worry if it is a painting by Ravi Varma or from a local press. What is more important that it's continued rememberance. For a sincere aspirant, God will manifest in the form that the devotee prays. Well, in the case of Prahlada, he even appeared in a form first of its kind, in which Prahlada would have never imagined!
Sri Ramakrishna is the best example of this, his chosen form was the Bhavadharini in the temple built by Rani Rasamoni in Dakshineswar, but that one form was enough him for him, so much so that, at one point he could feel the breath of the Goddess! It must be said, he also later checked out Christianity and had a vision of Christ, similarly with Islam. During one phase he worshipped Hanuman intensely and had experiences related to that one-pointed worship.
Similar is the experience of Meera, Andal or Arunagiranathar.
Counting the number of times or hours, is considered common in the initial stages of Namasmarana, and one can use a Japa Mala if you wish. But, Bhava or feeling is the most important, and over time, count and duration will drop on their own account, like the modules of a rocket that fall off to spur the rocket ahead.
Visualization : One could pick any favorite form or any favorite deity (and preferably, stick to it all through, everyday). One can visualize the form from the feet to the head. One can remember the Lord's Leelas. If you are chanting a mantra, one can dwell on the meaning of the mantra. In the Gita, Chapter 10, Krishna says :
The thoughts of My pure devotees dwell in Me, their lives are
surrendered to Me, and they derive great satisfaction and bliss
enlightening one another and conversing about Me.
The visualization is a powerful one. Consider these levels for evolution, in increasing order of intensity and importance :
The picture that we see outside is, Bahya Chitram, the picture without.
The picture that we see within is, Bhava Chitram, the picture within, reminiscing the Lord's attributes and leelas.
The picture that we internalize and experience within, without and everywhere in everyone , is, Sakshatkara Chitram, the transcendental form or the all-pervasive form.
The above gradation is explained by Sathya Sai Baba in one of his discourses. (p.s.Can't place the reference now, will have to add later.