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.