optimize images

correct issue from https://github.com/enyo/dropzone/pull/276
optimize images with optipng.

$ optipng -o7 *.png
** Processing: spritemap.png
428x406 pixels, 4x8 bits/pixel, RGB+alpha
Reducing image to 8 bits/pixel, 170 colors (166 transparent) in palette
Input IDAT size = 16860 bytes
Input file size = 16954 bytes

Trying:
  zc = 9  zm = 9  zs = 0  f = 0		IDAT size = 9444
  zc = 9  zm = 8  zs = 0  f = 0		IDAT size = 9444
  zc = 9  zm = 9  zs = 1  f = 0		IDAT size = 9414
                               
Selecting parameters:
  zc = 9  zm = 9  zs = 1  f = 0		IDAT size = 9414

Output IDAT size = 9414 bytes (7446 bytes decrease)
Output file size = 10208 bytes (6746 bytes = 39.79% decrease)

** Processing: spritemap@2x.png
856x812 pixels, 4x8 bits/pixel, RGB+alpha
Input IDAT size = 38411 bytes
Input file size = 41065 bytes

Trying:
  zc = 9  zm = 9  zs = 0  f = 0		IDAT size = 33248
  zc = 9  zm = 8  zs = 0  f = 0		IDAT size = 33021
                               
Selecting parameters:
  zc = 9  zm = 8  zs = 0  f = 0		IDAT size = 33021

Output IDAT size = 33021 bytes (5390 bytes decrease)
Output file size = 35675 bytes (5390 bytes = 13.13% decrease)