hm. in my opinion I don't think you should straighten the horizon- it looks cool how it is. makes the guitar look like it is part of the land, on a hill or a waterfall or something. and also, that way it takes a few seconds longer to register what the image is.
I wouldn't complain about anything. i love how the overhang (or whatever it is) goes up the left side of the piece, it gives it a sense of depth. The guitar (or whatever it is) gives it a new sense of originality, it seems fresh to me cause i've never seen anything like this.