I'm not very good with digital painting yet but I can comment on posing and construction. Greed and Lust have good posture thus far and it looks like you had some reference but Gluttony looks like fewer reference was used. This makes it look out of place compared to the more anatomically defined Greed and Lust. The silhouettes could be pushed further with each piece; I suggest working with line-of-action, especially gluttony.
When drawing more anatomically correct figures too, be prepared to do more detailed objects in a scene. The chair on which Greed sits is lacking the dimension you showed with greed himself. Whether that was a personal choice or not, I found it distracting from the overall piece.
Good things: You limited your colors with each creature, which is actually a common mistake. The colors weren't too saturated. You gave a background which is also extra work ( I do recommend more referencing to caves to give it that extra punch). Overall, a good beginning. I look forward to see improvement with your next four pieces.
Dooger had some great critiques. I like when style starts to show through in someones work. You put a lot of effort into the human form and they all seem a bit rigid especially in conjunction with the loosely composed backgrounds. I think that you would benefit from laying down contour gesture drawings with only a pen or brush so that you don't have the opportunity to correct yourself during this process keep your arms loose and draw with your arm. That exercise will help loosen up your drawings and gain more line confidence. These are two things that I struggle with in my drawings as well. I love the concept and am looking forward to seeing the followthrough on all 7 sins.