For the first picture, read the letters from top to down, left to right. FOrFor the second, turn it is upside down and do the same. The third needs to be rotated 90 degrees coutnercounter clockwise, the the fourth needs to be rotaterotated clockwise. Reading the letters, we get the message: THE CODE WORDS ARE HIDDEN FROM CORNER TO CORNER.
Read the diagonals of the first painting, which say: CRIMSON DIAMOND. I believe this is the description of the third painting.