Icons are a important part of any application so here is a few nice free collections I have come across that you might find helpful. You can add add a icon to a button like this:

Button {
    graphic: ImageView{ image: Image{ url:"{__DIR__}icon.png"}}

assuming the icon “icon.png” is in the same directory(package) as your source file.

Developpers Icons by Sekkyumu

Basic set by prokofusha

Free Vector Icon Set by monofactor

Lots more nice icons in this Smashing Magazine article 40 Professional Icon Sets For Free Download