Vajranava got three idols of Sri Krishna constructed and all three are in Rajasthan now.All three were recovered by the direct disciples of Mahaprabhu. But these images were worshipped without the image of Sri Radha.
None of the other deities worshipped by the direct disciples of Mahaprabhu had Sri Radhika's idol. All have been added after they left their bodies. (Reference: Hridi-Vrindavane, Swami Achyutananda, Udbodhan)
The dhyana-sloka, bija mantra and pranama mantras of Sri Radha are there in the Tantras and the Puranas, Srimad-Bhagavatam does not mention Her name directly though.
When and where was She first worshipped in the image form?