In Hinduism, every God is having its own image/form and characteristics.
Is there any God who having no form/image in Hinduism?
In Hinduism God has two forms or modes: Saguna Sakara (with form and characteristics) and Nirguna Nirakara(without form and characteristics):
dve vāva brahmaṇo rūpe, mūrtaṃ caivāmūrtaṃ ca [Brh. Up - 2.3.1]
-God (Brahman) has two modes, formless (nirakara) and form (sakara).
So even though we see God being worshipped in many forms, all of them do not have a form too. Just like our soul is without form but externally our body has a form, God is also both with and without form. It is only for our easy apprehension and concentration that we think of Him in many different forms. So the Bhagavatam also says thus:
iti mūrty-abhidhānena mantra-mūrtim amūrtikam
yajate yajña-puruṣaṁ sa samyag-darśanaḥ pumān [SB - 1.5.38]
Thus he is the actual seer who worships, in the form of transcendental sound representation, the Supreme Personality of Godhead, Viṣhṇu, who has no material form.
So even Lord Vishnu, whom we generally think of as four handed, is also worshiped without form. His four handed form has symbolic meaning.
Now the thing is, when God is considered in His formless and attribute-less form, there remains nothing to distinguish Him as Siva, Brahma or Vishnu. Only when we see some form or characteristics that we identify Him as a certain God. So the formless Brahma can be called by any name. But most often the name Siva is used to denote the formless quality-less Brahman:
śivaiko brahmarupatvānniṣkalaḥ parikīrtitaḥ [Shv. Pu. - 1.5.10]
- Shiva alone, being Brahman, is known as formless and quality less.
So the answer to your question is, every worshiped form of God (not demigods) is also devoid of form, but the mode of God which doesn't have any specific picture, idol or image is nirakara Brahman. But even then because we, as humans, have form, some worshipers of nirakara Brahman try to represent formless Brahhman with fire which also doesn't have a specific form.
At the Vishwanath temple in Sathur, Virudunagar, Tamil Nadu during the Tiruvadhirai festival in December they place Lord Nataraja in front of Viswanath and place a mirror before Lord Nataraja. Deepa aradhana is done to all three at the same time, namely, Siva in the Lord Nataraja form, formless as well as in the abstract form (linga) as if to indicate that Siva graces us in all these three forms.
If you search for Nirguna you should see several references, including:
"Saiva tradition identifies Lord Siva or Shiva as a formless eternal and mysterious being with many aspects and dimensions. He is both transcendental and immanent, who cannot be quantified and qualified objectively with our limited awareness. He is beyond our mind and senses, but within the reach of our experience and awakening."
Nirguna: "The Advaita (Nondualist) school of Vedānta assumes on the basis of selected passages of the Upaniṣads that Brahman is beyond all polarity and therefore cannot be characterized in the normal terms of human discursive thought. This being the case, Brahman cannot possess qualities that distinguish it from all other magnitudes, as Brahman is not a magnitude but is all. The fundamental text of this tenet is the Bṛhadāraṇyaka Upaniṣad definition of Brahman as neti-neti (“not this! not that!” 2.3.6). The scriptural texts that ascribe qualities to Brahman, leading to the conception of a qualified Brahman (saguṇa) are, according to the Advaita school, merely preparatory aids to meditations."
According to Advaita Vedanta God(Brahman)'s true nature is impersonal. Brahman is classified as Nirguna Brahman and Saguna Brahman or Ishwara.
Nirguna Brahman is the one and only absolute truth. Nirguna Brahman is formless, genderless, indivisible, eternal, infinite and changeless. Actually Nirguna Brahman cannot be described using words like 'infinite or 'eternal' as Nirguna Brahman is indescribable but for the sake of understanding of humans those adjectives are used.
The human mind can never think other than in human terms. It unknowingly projects human characteristics on Nirguna Brahman. Thus impersonal Nirguna Brahman acquires personality very much resembling a human personality. Impersonal Nirguna Brahman appears to become Personal Brahman, also called Saguna Brahman.
Hence God is actually formless and all deities and gods are Saguna Brahman i.e having human form as perceived by human intellect.
Swami Bhaskarananda, The Concept of God in Advaita Vedanta
Hinduism in its purest sense starting from the Vedas have a very complex to understand, but inherently very simple concept of God.
It is nothing and everything.
It is indescribable
It is eternal
The Self itself can reflect God
I am God.
So and so forth. I have tried to describe God using an analogy in another post, but let me get a crack at it again.
Now, I will do the argument in reverse
A multiverse consists for many universes A universe consists of many planets and other heavenly bodies A planet consists of many materials and lifeforms A lifeform conists of many cells A cell consists of many atoms and so on..
So, here there is a "contains" relationship.. Now, here is where we all seem to miss the bus.. Somehow, we have this collective mistaken notion that for something to be part of something else, it needs to be connected physically by a wire or tube or something else.. So, we being part of God, are not able to understand that we don't need a wire to be attached from us to God. We are only part of a larger construct. The same way, the electrons are simply floating around the nucleus, and not tied to the nucleus with a wire.
So, this is the image of God.. as articulated by core Hindu texts..
Now, for a open question.. If we are part of God, we have a couple of choices
So, many milleniums later, Hinduism must have felt that there is no particular reason to deny a personal image of God in anyone, given that the original theory ascribes a larger concept of indescribable nature of God. So, this creative license, I should day must have resulting in hundreds of people conjuring their own images of Gods. Artisans, Monks, Kings, going on an overdrive using their imagination to describe God in their own words and minds.
Now, after 5 millenium, in this forum , you are asking me why Hindus have so many Gods.. !!
The answer is : Hindus started with a very complicated description of God, as mentioned in the Vedas. And in that process, there must have been a compulsion to produce a more simpler, easier narration, and created so many diverse images, and descriptions of God.