The Art Gallery is a very simple but risky way to make money on the Island and here is why, even though the pieces themselves "could be" considered legal they can only be obtained illegally as you have to break in using "Bolt Cutters". You can buy a set of "Bolt Cutters" from any "Market" for "7,500". Once you have your bolt cutters you want to head over to the gallery, when the gallery is active you will see a purple "!" on your map, the Art will spawn in a warehouse. You simply walk over to any door and press on your windows key and select "Break In" you will then begin the breaking in process, once you are inside the building you will usually see "1-3 paintings" inside, you then approach the paintings and press "windows key" whilst looking at it to start picking them up. But be careful as when breaking in, it will send a message to the "Altis Police Service or Poseidon" depending on what side of the Island you are on. Once you have the paintings you will then proceed to the "Antique Dealer".