The Lost and Found achievement can be obtained in the Cultural Property scenario from the newly released Art of War charity DLC.

In the screenshot above is the locations of the first four paintings.

And in this screenshot is the location of the fifth, and last, painting.

The first painting is located in-between Point Charlie and the sniper at the start.

The second painting can be found just up the road next to a crashed helicopter.

The third painting can be found in a small wooden shed behind the wrecked building with the UGW command centre.

The fourth painting can be found in a blue container in the North-Western part of the town.

The fifth painting can be found in the woods just South of where the hostage is being held.