You can also use the font icon instead of the image.
CSS
.button { min-width:175px; border-radius:50px; background:none; outline:none; border:none; color:#fff; padding:15px 25px; margin-top:10px; background: rgba(115,84,178,1); background: -moz-linear-gradient(45deg, rgba(115,84,178,1) 0%, rgba(115,84,178,0.95) 34%, rgba(115,84,178,0.95) 35%, rgba(96,107,192,0.91) 62%, rgba(84,160,231,0.85) 100%); background: -webkit-gradient(left bottom, right top, color-stop(0%, rgba(115,84,178,1)), color-stop(34%, rgba(115,84,178,0.95)), color-stop(35%, rgba(115,84,178,0.95)), color-stop(62%, rgba(96,107,192,0.91)), color-stop(100%, rgba(84,160,231,0.85))); background: -webkit-linear-gradient(45deg, rgba(115,84,178,1) 0%, rgba(115,84,178,0.95) 34%, rgba(115,84,178,0.95) 35%, rgba(96,107,192,0.91) 62%, rgba(84,160,231,0.85) 100%); background: -o-linear-gradient(45deg, rgba(115,84,178,1) 0%, rgba(115,84,178,0.95) 34%, rgba(115,84,178,0.95) 35%, rgba(96,107,192,0.91) 62%, rgba(84,160,231,0.85) 100%); background: -ms-linear-gradient(45deg, rgba(115,84,178,1) 0%, rgba(115,84,178,0.95) 34%, rgba(115,84,178,0.95) 35%, rgba(96,107,192,0.91) 62%, rgba(84,160,231,0.85) 100%); background: linear-gradient(45deg, rgba(115,84,178,1) 0%, rgba(115,84,178,0.95) 34%, rgba(115,84,178,0.95) 35%, rgba(96,107,192,0.91) 62%, rgba(84,160,231,0.85) 100%); filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#7354b2', endColorstr='#54a0e7', GradientType=1 ); }
You can remove the background and filter part in the button class in the above style. I just used it for the gradient background for the button.
HTML
<button runat="server" id="btnDownload" class="button" onserverclick="clickEvent"> <i class="fa fa-download" style="font-size:20px;float:left;"></i> DOWNLOAD </button>
The button is as follows.

Screenshot here
And even you can change the color of the icon by simply adding color to <i>...</i> . Something like below.
<button runat="server" id="btnDownload" class="button" onserverclick="clickEvent"> <i class="fa fa-download" style="font-size:20px;float:left;color:#ff4444;"></i> DOWNLOAD </button>