You can purchase equipment upgrades and ammunition from Piero at the Hounds Pits Pub with any money you collect during missions. These equipment upgrades are things such as pistol accuracy, bolt reload speed, magazine size etc. You should attempt to purchase 15 equipment upgrades on the same playthrough you are collecting the Sokolov paintings on, as the paintings are worth a lot of coin.
This trophy is missable because all 15 upgrades have to be purchased in one playthrough by the time you reach the final mission. If you don’t have the trophy by then, you may need to attempt it on another playthrough. Try not to spend money on bolt/pistol ammo as that can add up really quick.