Graphic, resize

Hello, since im making my own game, Im wondering about resolution, re-sizing images, quality and so on.

Lately I have been checking 2D games and as i could see, they always fit perfectly.

So im wondering how do they manage to do it? Yesterday i have seen article about NinePatch, but its rather for buttons etc - (IM guessing)

Maybe graphic using Adobe Illustrator? I have really no idea.

Also if i want to put graphic in my project (in Graphical Layout editor, it always change size of the image)

Would be great if someone experienced may provide some information, thanks!