dedecms 5.1怎么判断缩略图有无

{dede:field name='officialurl' runphp='yes'} 
if(@me == "") @me = "未知官方";else @me = "<a href=".@me." target=_blank>官方网址</a>"; 
{/dede:field

{dede:field name='officialdemo' runphp='yes'} 
if(@me == "") @me = "无演示";else @me = "<a href=".@me." target=_blank>演示地址</a>"; 
{/dede:field}

{dede:field name='litpic' runphp='yes'} 
if(@me == "/do/plus/img/dfpic.gif") @me = "";else @me = "<a href=".@me." target=_blank title=新窗口查看图片><img src=".@me."  /></a>"; 
{/dede:field}