Screenwidth and screenheight variables are the best x.X Don't use playerx and playery positioning, because when you are inside or you go to any side of a level, and you're not in the middle of the screen, it bojangles it all to hell
PLUS that is set for your screenwidth/height. Most people don't resize, but it's always nice to make it compatible. Using screenwidth/height does not use a deffinative value, instead it's a variable. Look into it. When dealing with screenwidth/height you ALSO have to change the img's visiblity to 4 or more.
---Shifter