There are numerous bases for the idol worship in the scriptures. To learn it fully, first try to understand NAVADHA BHAKTI (devotion that has nine rituals.)

SHRAVANAM KIRTANAM VISHNOMAH SMARANAM PAD SEWANAM |
ARCHANAM VANDANAM DASYAM SAKHYAMATMA NIVEDANAM ||

Meaning : Devotion to the Lord Vishnu has nine types : Shravana (hear), Kirtana (devotion songs), Smaran (remembrance), Pada Sewan (touching his feet), Archana (worship his idol), Vandana (Prayer), Dasya, Sakhya and Atma Nivedana. Among these nine kinds at the fifth place is Archana - that is idol worship. Thus it occupies the central position in all the nine types. As such it must be taken as the hub of the devotional cycle, and all other forms as the circumference like the parts of a wheel that face upward or downward, other parts of the devotional wheel face upwards or downwards. The first four parts of devotional wheels like hymns, remembrance, etc. are meant for the common devotees, while the later four parts like prayer etc. are meant for the devotees of the higher class. These classes of devotees can be arranged ina sequence, yet no one of them is less than the other as far as the goal is concerned. Thus to practice the Archana kind of devotion, idol worship is necessary, for without an idol, visual worship is impossible. Hence idol worship is must, even though it is for practice. Scriptures like Vedas also describe in detail the virtues of idol worship.

VEDIC DESCRIPTION

1. MA ASI PRABHA ASAPRATIMAASI (TATTI)

2. ETU PRANA ETU MANAHETU CHAKSHORATHO BALAM (ARTHAVA)

3. SAHASTRASYA PRATIMAASI (YAJURVEDA)

4. ARCHATA PRARCHATA PRIYA MEDHASOARCHATA (RIGVEDA)

Since the begin of human tradition, many great souls have achieved salvation through idol worship. It is difficult to the fire these events chronologically. But, as is evident in the scriptures, in satyuga (the Manvantar before that could also be taken as Satayuga) having preached by Narada, the boy Dhruva got the sight of almighty God within six months through idol worship. In Treta, Lord Rama established a Shivalinga at Rameshwaram, which still exists there in modern age. According to Valmiki Ramayana the demon king of Lanka, Ravana used to carry a golden Shivalinga in his chariot and worship it regularly.
In the great epic Mahabharat also, there are many examples of idol worship. Prominent of them, is the example of Ekalavya. After having denied by Guru Dronacharya of archery. Ekalavya did not lose his courage. Keeping full faith in Guru, he made an idol of Guru Drona using soil and learned such an excellent archery from it that he surpassed even Arjuna in his skill. From these scriptural examples, it is clear that through idol worship one can get his desires fulfilled.

EFFECT OF IDOL WORSHIP IN MODERN AGE

The almighty God has two forms, First is Saguna (With a solid form) and second is Nirguna (Without a solid form)

Saguna Sakara - God is conceived in solid form having all the organs and parts like - hands, feet, mouth, eyes, ears etc. with all the physical virtues. This is the basis of idol worship. In idol worship, faith and devotion are the feelings of the supreme order. Without these feelings it is difficult to see the diginity of God. But once these feelings take firm root, nothing remains impossible and there exists no difference between God and the Devotee. One great example of this assertion is Meerabai. Devotion of Meerabai is so strong, so powerful that, in the end, she merges in the stone idol of Lord Krishna. There is another tale of Tulsidas that says : Once Tulsidas went to Vrindavana, where he entered a temple seeing the idol of Lord Krishna, there he said

TULSI MASTAK TABA NAMAH |
DHANUSHA BANA LO HATHA ||

KRITA MURLI KRITANDRIKA, KRIT GOPIYANA KE SATHA|
APNE JANAKEKARANA, SHRI KRISHNA BHAYO RAGHUNATH ||

Meaning : At the insistence of the devotees the idol of Shri Krishna changed into the archer Rama, Thus, because of the mental determination of a devotee, form and appearance of a stone idol can change. These evidences, amply support the ritual of idol worship and its significance.

To concentrate his attention, a human being requires some kind of purpose. Without purpose, concentration of mind is difficult. It is also not wise to fix some physical entity as a purpose, because the physical entity is a means of pleasure are short lived and mortal. Hence, only the ommipotent eternal Sachchidananda form must be set up as the purpose. That Sachchidananda form needs to be given a tangible appearance. This prerequisite cant be fulfilled without an idol. Need of idol worship hence becomes apparent.