Suggestions
Problems I see with the image, since you ask:
1. Anatomy- the faces. The bodies are close enough to okay, but the faces need work. Harley's lips need to be redone so they look human, her ear is a bit far up, and it looks like it's got a bunch of different but similar perspectives jammed together. Both faces could use more modeling. Jokers' hand also needs work- don't forget to put in the bump where the ulna (wrist bone) sticks out a little. Try looking at your own hand in a similar position with similar lighting while you draw it- hands have a great reputation for being impossible to draw, but they're not so difficult when one has a model to refer to.
2. Composition- if you move joker a little to the right (you might have to make the image a little wider) you'd have a nice triangular composition formed by harley's head, the gun, and joker's head, with harley's thighs leading the viewer's eyes around. As it is things feel a little cramped.
3. bit of text on harley's leg- doesn't look like a tattoo, it's too bright and doesn't conform to the leg as much as it should. Try putting it on so that the tips of the letters are out of view (like you did with the heart and spade- quite effective), or better yet, just get rid of it.
4. Try giving Harley some black boots so that her legs don't seam to be being eaten by the chairs- I'm not sure whether the red things in front of her knees are meant to be boots or chairs, and it would be clearer if you change the color.
Also, the jacket's fine, and you've got a great color palate going. Good luck.