I'm always thinking how Lord Krishna is seen different in every photo , sculpture idol etc . Was there anyone who drew the first painting of him or what we see everywhere is only imagination?
It is not required that some one must see a God first before drawing his image.
When, Gods are essentially formless, how they will look, when they assume a form, are mentioned in the Dhyana Shlokas (meditative verses) of that particular deity. Dhyana Shloka is chanted , that's why, at the very beginning of a Puja. Such Dhyana Shlokas are found in many scriptures.
Dhyana Shlokas are elaborate descriptions of not only how the deities' physical features will be, but they also mention the details of the deity's garments, ornaments, mounts and also the weapons they carry in their hands.
If the Dhyana Shloka is very detailed, it is not that difficult to draw an accurate image of the deity from it.
For example, from Lord Shiva's Dhyana Shloka:
Santham padmasanastham sasa dhara makutam panchavakthram trinethram,
Soolam vajram cha gadgam parasumabhayakam daksha bhahe vahantham,
Nagam pasam cha gandaam pralayahuthavaham sangusam vama bhage,
Nanalangara deeptham sphatikamani nibham parvatheesam namami||
I salute the consort of Goddess Parvathi, Who is peaceful, who sits in a lotus pose, Who wears the moon on his crown, Who has five faces, who has three eyes, Who holds Soola, Vajra, white axe, sword And symbol of protection on his right, Who holds snake, rope bell, fire of deluge And goad on his left side, Who shines by his several ornaments, And who has a luster of the crystal ball.
From this description one can visualise how Lord Shiva will look and draw a picture accordingly.
Similarly, from Goddess TripuraSundari's Dhyanam:
Balarkayutha thejasam trinayanam rakthamarollasineem,
Nanalangruthirajamana vapusham baloduratchekaram,
Hasthairikshadhanusyanim sumasaram pasam mudhra bibrathim,
Sri chakra sthitha sundarim trijagathamadara bhootham smareth.||
I meditate on her who is the basis of all the three worlds, Who shines like the rising sun, who has three eyes, who dresses, Herself with cloths of blood red colour, Who shines in a pretty form Which shines with different ornaments, who wears the crescent on her head, Who holds a bow of sugarcane, flower arrow, rope and goad in her four hands, And who is the pretty goddess who sits on the Sri Chakra.
Besides, these two examples, which are quite basic in nature, there are more Dhyanams for the same deities which are much more detailed and following which it is not much difficult to create an idol or image of the deities. So, as I said at the start, it is not that if you haven't seen a God you can't know how he looks like.
Also, after attaining Mantra Siddhi by doing Purascharana, one can attain the vision of that deity too. This is mentioned in the scriptures.
For example, as shown in this answer, one of the signs of Mantra Siddhi, is Devatadarshana or vision of the deity.
Some saints from the Swami Shivananda's Asrama have also recorded their such experiences in their website. Will update this answer once I am able to locate that page.