how do I get my shelf wallpaper to fit my icons??

It may be zoomed in because of the Parallax.

Go to the set wallpaper screen.
Turn off parallax
Then pinch out the picture so it returns to normal size
Set the wallpaper.

 

If you are using a 6/6+, is the app updated for 750p and 1080p wallpapers?

 

You could try here I've had a couple shelf wallpapers rebuilt from the ground up to fit my iP6.

http://forums.macrumors.com/forumdisplay.php?f=161

 

Download Wallpaper Fix, that is if you are actually attempting to resize ANY image to fit your device display.