Check box icon from <a href="https://en.wikipedia.org/wiki/The_Noun_Project" class="extiw" title="en:The Noun Project">The Noun Project</a>.
<a rel="nofollow" class="external text" href="http://thenounproject.com/sutterlity">Laurent Sutterlity</a>
2013
<a rel="nofollow" class="external text" href="http://thenounproject.com/noun/check-box/#icon-No10759">The Noun Project</a>