>> img = imread('football.jpg');
imread to read image files into a matrix in MATLAB.
imread an image, it is stored as an ND-array in memory:
>> size(img) ans = 256 320 3
'football.jpg' has 256 rows and 320 columns and it has 3 color channels: Red, Green and Blue.
You can now mirror it:
>> mirrored = img(:, end:-1:1, :); %// like mirroring any ND-array in Matlab
And finally, write it back as an image using
>> imwrite(mirrored, 'mirrored_football.jpg');