样式：

<style>
    .bshApp img:hover{
      content:url('bshsjjscpdf/bsh_four_bottom/btn_grd_hover.png');
    }
    .bshWeb img:hover{
      content:url('bshsjjscpdf/bsh_four_bottom/btn_jkglpt_hover.png');
    }
</style>

div

<div class="four-bottom">
    <div class="firstLine">
       <div class="bshApp ">
            <img class="bottomImg" src="bshsjjscpdf/bsh_four_bottom/btn_grd_normal.png"> 
       </div>
       <div class="bshWeb">
            <img class="bottomImg" src="bshsjjscpdf/bsh_four_bottom/btn_jkglpt_normal.png">
       </div>
    </div>
</div>

默认：

悬空：