The easiest way to convert images to grayscale ( graylevel ) is by using "imagemagick" the image manipulation programs suite. Use the commands below to install imagemagick installed on your Linux system if you have not done so yet:
# yum install imagemagick
# apt-get install imagemagick

Convert Image to grayscale

The procedure to convert color to grayscale it simple. Here is our original color image which needs to be converted to grayscale:

now we convert the above image to grayscale:
$ convert linuxcareer-color.png -colorspace gray linuxcareer-color-grayscale.png