Goswami Tulsidas once said that there are certain mysteries of life which are best not revealed to others. This is hundred per cent true about mantras. Mind is the basis of mantra. Mantra is a medium to take positive message to the mind. The mantra doesn't have any effect of its own as long as you don't have faith in it. Miracles happen as soon as faith is expressed. Like you can't do anything with a mobile phone set with no SIM card in it. SIM card is like faith that enables the device to connect you worldwide once it's inserted. Perhaps that's why people don't connect to God in spite of reciting the name for their entire life, whereas, Valmiki reached and became equal to Brahma even through he recited it in reverse order.
That is why, the tradition of whispering the mantra in the ear of a disciple by the Guru and maintaining secrecy about it began. Only the Guru knows the reality, as he knows the purpose of giving that mantra. Therefore, the same photo, rosary, mantra, procedure etc could be divine for one person while it may be insignificant for others. It all depends on their faith.
Faith means turning your life energy and mental power in a particular direction so it does not get diffused by doubts and perplexity. With faith, mind becomes Urdhawagami (travelling upwards) and with doubt, it becomes Adhogami (traveling downwards). If there is faith in one's mind, one can achieve the impossible; otherwise, a doubtful soul renders one incapable of accomplishing even the most ordinary task.
No matter in what form the mantra is, its purpose is always human welfare. That is why, in almost all religions, a mantra enjoys a significant place. Only the Guru decides what you should know about the mantra and what you shouldn't, because it's not the mantra, but its presentation that is important.
Human brain is very subtle and complex and false information diminishes its efficiency, which is called wrong notion. Even the most intelligent, successful and rich people fall prey to wrong notion, which is like a fault in the circuit obstructing mental development. But with guidance from a capable Guru, the mind's spectrum broadens.
Many people come to me who often read about serious ailments in detail over the internet and start feeling that they are also suffering from those diseases and keep paying hefty fees to super-specialist doctors; but due to the lack of faith, they cannot get rid of those imaginary diseases. At the same time, some doctor or Guru or his loved one in whom he has full faith wins his confidence and makes him well, even if he is not a specialist. In fact, the disease has to be removed from the mind, as it never exists in the body. If that is not done, the mind being so powerful can create imaginary diseases in the body that in fact don't exist.