create image whose file size you can specify for test

create_image.rb

#
# brew install imagemagick exiftool
#
system('convert -size 128x128 xc:blue test.jpg')
file_size = 1_048_576 - 448

ascii = ('a'..'z').to_a + ('A'..'Z').to_a + ('0'..'9').to_a

File.open('random.txt', 'w') do |file|
  file.write(file_size.times.map { ascii.sample }.join)
end

puts `exiftool test.jpg -comment\\<=random.txt`