@-moz-keyframes backchange {
      0%   {background-image:url(../img/back-bl.png)}
      49.5%   {background-image:url(../img/back-bl.png)}
      50%   {background-image:url(../img/back-gr.png)}
      99.5%   {background-image:url(../img/back-gr.png)}
      100% {background-image:url(../img/back-bl.png)}
    }

    @-webkit-keyframes backchange {
      0%   {background-image:url(../img/back-bl.png)}
      49.5%   {background-image:url(../img/back-bl.png)}
      50%   {background-image:url(../img/back-gr.png)}
      99.5%   {background-image:url(../img/back-gr.png)}
      100% {background-image:url(../img/back-bl.png)}
    }

    @keyframes backchange{
      0%   {background-image:url(../img/back-bl.png)}
      49.5%   {background-image:url(../img/back-bl.png)}
      50%   {background-image:url(../img/back-gr.png)}
      99.5%   {background-image:url(../img/back-gr.png)}
      100% {background-image:url(../img/back-bl.png)}
    }