Answer
You have asked about Manasa Puja, or mental worship. Just as mental japa is considered superior to vocal japa and even whispered (upamshu) japa, so too, in truth, mental worship is superior to the worship of a physical idol.
One reason for this is that the possibilities in mental worship are limitless. The flower that you ordinarily cannot offer to the deity because it is not available in your village or city -- such as a blue lotus (nila kamala) -- even that flower you can offer at the Lord's feet in mental worship.
Another point is that if you desire Saguna Moksha -- that is, after death you do not wish to lose your existence and individuality by dissolving into the formless Brahman, and instead wish to go to Vaikuntha and reside there for all eternity, free from the cycle of birth and death, enjoying the bliss of Vaikuntha -- then you can, while seated in meditation, go to Vaikuntha and directly offer worship at the Lord's feet there. No temple on earth will be as beautiful as that. Where Narayana Himself is seated in His four-armed form -- what idol could compare? So whenever you find the opportunity -- there is no rule for this, no fixed time -- sit in meditation, reach Vaikuntha, and perform direct mental worship of the Lord there. Offer all manner of gifts, present flowers, place morsels of food directly into His mouth with your own hands. You will remain absorbed in this for a very long time, provided no one calls you in between.
In this form of worship, hours pass by. If you go to a temple to worship, there will be great noise and commotion, jostling and pushing. But for this practice, you will need to form a mental picture of Vaikuntha. The Bhagavata Purana contains a depiction of Vaikuntha; the Vishnu Purana also has one, as do other texts. If anyone among you wishes, I can send a foundational word-picture of Vaikuntha. But you can also imagine something even more beautiful, creating your own Vaikuntha from your own mind. Try it -- it will bring great joy and your heart will be deeply satisfied. This is the very delight of bhakti! Hari Sharanam!