I am going to take a crack at a critique here
I will be coming at this from a photographer side of things. I will start with a short critique of each image and then try and give some general advice for editing.
Justice Shall Prevail
This one isn't doing much for me. I think that first and foremost comes down to the composition. Putting your subject directly in the center like that doesn't exactly make for an exciting image, but instead it feels very static. The fact that the photo is taken straight on to the cosplayer and makes the image almost symmetrical doesn't help that either. Coming at this shot from a bit of an angle will probably bring a lot more life to the image.The next problem comes in the form of blown highlights. The details on the feet, the knee, and the chair and completely gone and are pure white. You could have brought your exposure down, but that would have given you very dark shadows. What I would suggest would be to try to soften the light. It looks like it is coming from a window, so maybe if it has white curtains, draw them shut. The last thing about this image that bothers me is the hand near the face. Because of the straight on angle, it is really hard to see what it is doing there, I can't quite tell if the thumb is on the lip or not. Telling them to bring their hand down a little lower will help with this (I think).
Right off of the bat, this shot is underexposed by a 1.5-2 stops...it is just too dark. Bumping that up will REALLY help with this shot (and you could even do that to some extent in post). I would maybe pull the framing back a little bit as well to get the top of the head a little breathing room as well as fitting all of the left hand in to the frame. I would also bring a wig comb to your shoot, cause god know that wigs always get messed up and fixing it in post is a major pain.
Dump the fake lens flare, it isn't doing your shot any favors.
I would also consider not going for a dutch angle...but this is just personal preference, I find them to look very amateur unless you have a very good visual reason to them them (Which means I really don't mind them if I can't see the horizon line). Framing wise, I think this is the strongest of the bunch.
Here's my quick edit of what I would do in photoshop:
1) Fix the horizon line (I used content aware fill to fix the empty corners)
2) Color balance adjustment layer to introduce a bit more reds and yellows
3) Use a gradient map adjustment layer with an orange tone and use a layer mask with a large soft brush to limit it to the upper right corner.
4) Use a soft white brush in the upper right corner to give it a "realistic" flare)
5) Use a levels adjustment layer to bring up the black level to make the image appear "softer"
End result: http://oi39.tinypic.com/2s0zif6.jpg
Monkey D. Luffy
Again with the dutch angle…I don’t think it is adding anything to the picture here and I think it would be better off with the horizon straightened. I like the pose on this one, and I think if you cropped in on the left it would be a lot stronger of a composition. Also, the cosplayer’s face is very much under exposed, all the shadow detail is lost right under the hat.
As far as editing tips go, I would like to know what equipment and software you have first. If you have a camera that can take pictures in raw, make sure that you are using that. I do all of my editing in photoshop and lightroom, so those are the only programs I can speak specifically to. Overall, I just think you need to work on getting a bit cleaner of an image, keeping the highlights and shadows in check, and then playing around with the color balance.
I hope some of those comments helped. And please, don’t take anything I say as fact, they are just my opinions and observations…someone might pop down after my comment and say that dutch angle are awesome lol.