Whenever I try to add a render of a person to a signiture, it always turns out wrong, sort of forced, like their just floating in space, with nothing to stand on. Is there anyway to correct this?
Don't put the whole of the character in. You shouldn't be able to see their feet. They're not actually in space, they're just there to represent the character as a part of the sig. Stopping at the waist at the border of the sig is usually a good guideline.
THEY TOLD ME I COULD BE ANYTHING SO I BECAME A SIGNATURE PLS HLP
There are also several ways to tie a character into the scene you're pasting into.
One, blending. Make sure that the outline of the character doesn't make him/her pop out of the scene (unless that's what you're going for)
Two, shading/highlighting. Character shading and highlights shouldn't contrast too sharply from the picture that they're being added to. For example, you wouldn't want to use a dark brooding monk in black on top of the heart nebula. Just don' work.
Three, my favorite, color. Hide bits of color from your background in your character, especially in the shadow. Does your background have a lot of purple in it? Add a touch of purple to the character's shadows and they'll feel more in place. Lighter colors can be added to highlights and the like. Don't go overboard with this too much, this is another place where less is more.
Those are my tips. I'm sure there are others. If you can provide an example, I'm sure people would be able to help you more.