Question
Does any God exist without form/image in Hinduism?
Answer
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]Meaning
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.
Note: “The question: Does any God exist without form/image in Hinduism?” is licensed by Stack Exchange Inc (https://hinduism.stackexchange.com/); user contributions licensed under CC BY-SA.