This is excellence! Really great work! I am a huge pinup fan and this is beautiful! Bettie's leg in the back with the seam and the gradation of values is superb. The only minor critiques I could offer would be on the woman doing the spanking, her hand is more defined than many other aspects of the painting so it draws the eye and her eye could seem the very slightest open (not really open just the slight thicker darker section at the front) and the eyelid maybe a little more angled like the reference to give it a little more "life" in her and less like she could be sleeping. These are very minor though, seriously, the work you have done here in such a limited color palette is stunning.
Also, I have to thank you because I was trying to google the ref image to see it bigger to give a better critique and while I never found it I found there are tons of pictures of Betty being spanked, and she was one of the sexiest things in the world so that was a great bonus!