Create pixel art!

Sledgehammer, 22 17 bytes

^{I noticed I'm no longer winning for some reason and made a minor IO golf}

⢟⢡⡂⠴⠒⢂⢜⠧⣘⡨⡏⣻⢈⠯⣧⠼⡫

Corresponding Mathematica code: Export[".bmp",ImageAdjust[Import[#]~Downsample~2, 9!]]& . Takes input from the file with name specified in the program's arguments (for some reason these are put in a file, too), outputs to the file .bmp. There are literally built-ins for everything!

• built-in for reading arbitrary image format
• built-in for downsampling
• built-in for adjusting the contrast by \$9!\$
• built-in for writing all that into a file

At least it's not PixelArtify[Input[]].

Mathematica, 61 31 bytes

ImageAdjust[#~Downsample~2,9!]&

Adjusts the contrast of the downsampled image by the factorial of nine.

MATL, 18 bytes

Yi2Lt3:K$)127>o2YG

The input is a string with the file name. The output is an image displayed in a window.

Input image:

Output image:

Explanation

Yi    % Implicit input: filename. Read image. Gives an N×M×3 uint8 array
2L    % Push [2 2 j] (predefined literal). When interpreted as an index,
      % this means 2:2:end
t     % Duplicate
3:    % Push [1 2 3]
K$)   % 4-input indexing. Downsamples the image by a factor of 2 in each
      % dimension of the first two dimensions (vertical and horizontal),
      % while keeping the three colour components
127>  % Greater than 127? Gives true (1) or false (0)
o     % Convert to double
2YG   % Display image. For double data type this assumes range from 0 to 1

Python 3 + imageio, 68 67 bytes

Takes the filename as input, overwrites the original file.

from imageio import*
lambda f:imwrite(f,(~imread(f)[::2,::2]>>7)-1)

imageio.imread returns a numpy 3d array of unsigned 8-bit integers corresponding to the RGB value of each pixel. array[::2, ::2] takes every other row and every other column of the array.

Because of the 8-bit data type (~array>>7)-1 is equivalent to ((255-array)//128-1)%256.

image source