Which preposition to use with backdrop

for Occurrences 5%

Another time I accompanied Ashok to a client's office to put a pair of Dwarf Guramies in the fish tank and to fix a picture as a backdrop for the tank.

of Occurrences 2%

Alemao's criticism (with some validity, even if ironical in the backdrop of his own exclusivist approach which sought compltely illegitimise the Marathi demand, in what was in is more of a caste-defined battle) was that the headline was not justified when the dialect and script used by a small minority had been given official acceptance.

Which preposition to use with  backdrop