well..if IU is smiling in this picture it would be great but you did edit it better than me. haha i'm not good at this edit thing but you did a great job

Just a few tips. When you're making a cut-out (png/render/whatever you wanna call it), I suggest using the pen tool. It just makes the cut-out come out a lot cleaner.

As for making IU blend in more, you're gonna have to tweak her colors a bit. You can do this by using the curves setting, or using photoshop actions, or PSD actions, etc. Also, notice how the light is shining down on the center (focusing mainly on Sehun). Basically what I'm trying to get at is to pay close attention to where/how the shadows are cast.

Tumblr has a bunch of great resource blogs that you can to go. My personal favorites are itsphotoshop and yeahresources.