Insert 255 px image into multi-resolution icons #17

added ico icon which support 16X16 32X32  48X48 256X256 with help of  Magick ImageMagick® is a software suite

convert plot_file_16.png plot_file_32.png plot_file_48.png plot_file_256.png -colors 256 plot_file.ico

Testing :
convert plot_file.ico plot_file_256.png 
convert plot_file.ico plot_fil_256.png generates plot_fil_256-0.png, plot_fil_256-1.png, etc

An alternate testing strategy is to view the icons in MS Windows Explorer, then successively select "small icons", "medium sized icons", "large icons" then "extra-large icons".