ICO is actually a special image file format and file should be saved in that format. Starting with a GIF or PNG file and renaming it as some sites recommend has not always worked with all browsers.
It is better if you use image editing software that works with the ICO format and convert the file. Both Gimp and Irfanview can save an image in the correct format. It looks like Paint.net is Gimp, so that makes sense. Paint Shop Pro and many other image editors cannot work with ICO files.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin