Shivji is devoted to Shaivism itself, he is his own first votary, so he salutes his own higher self. This is made clearer elsewhere in photos where Shiva is shown praying to a Shivlinga. Also he knows that Parvati, who is his devotee, is in fact Devi Sakti, to whom he is devoted!!

Somewhere in King Himavat's mountain range...
Extremely beautiful depiction of a rare moment.
Parvati attends upon an ascetic Shiva
whilst he makes her father's Himalayas his abode.
Now the question, who does Shiva pray to with folded hands?
First, Shivji is devoted to Shaivism itself, he is his own first votary, so he salutes his own higher self. This is made clearer elsewhere in photos where Shiva is shown praying to a Shivlinga.
Also he knows that Parvati, who is his devotee, is in fact Devi Sakti, to whom he is devoted!!
There is actually no difference between Shiva and Shakti like fire and its warmth, but one can always salute the immanence of one's own play, whilst remaining absorbed in transcendence.

{Note: this timeline is after the death of Sati, it is said Shiva was so distraught, he wanted nothing to do with the world, he wanted none of the mingling and recognition that was His at Kailash.
And he went away to a different part of the Himalayas.
Where King Himavat, who ruled, greeted him warmly, and made him an honoured resident guest. Shiva Desired to be left alone to His meditations, His solitude of remaining immersed in the thought of his beloved Sati. But who would stop Parvati who had already taken birth as King Himavat's daughter from attending upon herLord Shiva!}