I checked the verse given in the question. It has nothing to do with idol worship.
andham tamah pravisanti yesambhutim upasate tato bhuya iva te tamo ya
Into blinding darkness enter those who worhip the unmanifest and into
still greater darkness, as it were, those who delight in the manifest.
Isa Upanishad 12
asambhuti: the unmanifest, the undifferentiated prakrti. We get our
rewards according to our beliefs.
asambhuti: non-becoming: Those who
do not believe in re-birth may be referred to.
sambhuti: the manifest,
the lord of the phenomenal world, karya-brahma, Hiranya-garbha.
Sankara [Adi Sankara's interpretation of sambhuti]. It is sometimes said that asambhuti means that the world has
no creator, that it is produced, preserved and destroyed by its own
nature. Those who hold such view are the naturalists. See Bhagavad
The Supreme is neither of these in the sense that he is not also the
other. If we identify the Supreme with the manifest, it would be
pantheism in the sense that the whole of the Divine nature finds
expression in the manifested world, leaving nothing over, and it is a
wrong view. Again, if the world of becoming were not there, it would
also disappear in what would seem a world of undifferentiated
abstraction. Within the depths of the spirit there is unfolded before
us the drama of God's dealings with man and man's with God. Unity and
multiplicity are both aspects of the Supreme and therefore the nature
of the Supreme is said to be inconceivable.
The Principal Upanishads, Isa Upanishad 12, Commentary by S. Radhakrishnan
One thing they say, is obtained from the worship of the manifested;
another they say, from the worship of the unmanifested. Thus we have
heard from the wise who taught us this.
Isa Upanishad 13
One thing etc.: As a result of worshipping Hiranyagarbha, the devotee
obtains such supernatural powers as the ability to reduce himself to
the size of an atom, to make himself light as a feather, and to
pervade the entire universe. These are characteristics of
Another etc: According to the Puranas, the result of such worship is
absorption in prakriti.
The scriptures say that whatever a person worships he becomes after
death. He who worships Prakriti, characterized by nescience, remains
merged in prakriti for untold years. And he who worships
Hiranyagarbha, who is only a manifestation of Prakriti, obtains a
Isa Upanishad 13, commentary of Swami Nikahilananda
Basically you will get powers by worshipping Hiranyagarbha but will remain at the level of Prakriti. Hiranyagarbha is a manifestation of Lord Brahma at the level of Prakriti. Worshipping Lord Brahma as pure Consciousness will lead to moksha while worshipping Hiranyagarbha will lead to complete mastery of Prakriti.
Here are two examples of scripture supporting Murthi puja.
Till the realisation of perfect knowledge a man should continue the
ritualistic worship of Shiva. 59-60. In order to convince the world,
the rituals must be continued.
Just as the sun is reflected in many vessels, in the same manner, O
devas, know that the supreme Brahman, Shiva, assumes the form of
whatever is seen or heard in the world real or unreal. There is
difference in vessels but not in the water they contain.
This is what those who know the real meaning of the Vedas say. "Lord
Shiva is within the heart of beings in this world." Of what avail are
images to those who have the real knowledge?
Having an image is very auspicious for a person who has no such
knowledge. It is a ladder that enables him to climb to a higher
It is very difficult to climb to a position without a support. The
image is only a means to achieve the Nirguna Shiva.
The attainment of the Nirguna through a Saguna is certainly possible.
In this manner, the symbols of all lords are conductive to steady
faith and belief.
This lord is very great and this is the mode of worship of that lord.
If there is no image, of what avail are scents, sandal paste, flowers
Till the realisation of true knowledge, the image shall necessarily be
worshipped. If any one does not worship the image before he attains
perfect knowledge, his downfall is sure.
Shiva Purana, Rudra Samhita, Section I, Creation, Chapter 12
Whoever makes an offering to Me with devotion, be it of leaf, flower,
fruit or water - that devout offering made by a pure-hearted man, I
accept with joy.
Where would you put these offerings if there is no murthi or image?
There is also warning not to regard a murthi as mere stone.
To perdition he goes who regards the Guru as human, the Mantra as mere
letters and the Images as stone.
Kularnava Tantra, Devotion to Guru, Readings by M.P. Pandit