You need to first convert the image you want as an icon to a .ico file so that it can indeed be used as an icon. There are a lot of programs that are commercial for what is really such a trivial task, so here, let me link you to a free one:
Basically, .PNG is a standard image file format that most image editors can save as, make sure you save the image you want as an icon as a PNG image rather than a JPEG. Then upload it on to that site and you will be given a .ico image as a result. Then all you need to do is set the icon in the project's settings using the icon file yielded by that website.