Edit in JSFiddle

<div class="logo item1"><a href="http://caibaojian.com"><img src="http://caibaojian.com/wp-content/themes/c2/images/logo.png" width="138" height="30"/></a></div>
<p>第一个在IE8中不出现:因为父类浮动、图片内设width，css图片设为max-width:100%;</p>
<hr/>
<div class="logo item2"><a href="http://caibaojian.com"><img src="http://caibaojian.com/wp-content/themes/c2/images/logo.png" width="138" height="30"/></a></div>

<p>第二个可以看见：因为设置了图片的父类浮动的宽度</p>
    <hr/>
<div class="logo item3"><a href="http://caibaojian.com"><img src="http://caibaojian.com/wp-content/themes/c2/images/logo.png"  height="30"/></a></div>

<p>第三个可以看见：因为去掉了图片的width属性</p><hr/>
    <div class="logo item4"><a href="http://caibaojian.com"><img src="http://caibaojian.com/wp-content/themes/c2/images/logo.png" width="138" height="30"/></a></div>

<p>第四个可以看见：因为设置了图片CSS里面的width:auto;</p><hr/>

<div class="logo item5"><a href="http://caibaojian.com"><img src="http://caibaojian.com/wp-content/themes/c2/images/logo.png" width="138" height="30"/></a></div>

<p>第五个可以看见：因为设置了图片CSS里面的max-width:none;</p><hr/>

img{max-width:100%;}
.logo{float:left;}
hr{clear:both;}
.item2{width:138px;}
.item4 img{width:auto;}
.item5 img{max-width:none;}