declare @tb table (s varchar(1000))
insert into @tb select '<P>  </P>   <P>123 </P>   <P>12 <IMG height=25 src="/WebSite/upfiles/2721.gif" width=50 border=0> </P>   <P>312 </P>   <P>312'


select substring(s,patindex('%src="%',s),charindex('"',s,patindex('%src="%',s)+5)-(patindex('%src="%',s))+1) from @Tb
where s like '%src="%'

------解决方案--------------------
SQL code
if object_id('tempdb.dbo.#') is not null drop table #
create table # (c varchar(8000))
insert # select '<P> src= </P>   <P>123 </P>   <P>12 <IMG height=25 src="/WebSite/upfiles/2721.gif" width=50 border=0> </P>   <P>312 </P>   <P>312 </P>   <P>  </P>'
insert # select '<P> src=" </P>   <P>123 </P>   <P>12 <IMG height=48 src="/WebSite/upfiles/2696.gif" width=40 border=0> </P>   <P>312 </P>   <P>312 </P>   <P>  </P>'
insert # select '<P>  </P>   <P>123 </P>   <P>12 </P>   <P>312 </P>   <P>312 </P>  </P>'

select * from #

select substring(c, charindex('src=',c,charindex('<IMG',c)), charindex('"',c,charindex('src=',c,charindex('<IMG',c))+5)-charindex('src=',c,charindex('<IMG',c))+1) from # where charindex('src=',c,charindex('<IMG',c))>0
/*
src="/WebSite/upfiles/2721.gif"
src="/WebSite/upfiles/2696.gif"
*/

------解决方案--------------------
SQL code

create table # (news_content varchar(8000))
insert # select '<P>  </P>   <P>123 </P>   <P>12 <IMG height=25 src="/WebSite/upfiles/2721.gif" width=50 border=0> </P>   <P>312 </P>   <P>312 </P>   <P>  </P> '
insert # select '<P>  </P>   <P>123 </P>   <P>12 <IMG height=48 src="/WebSite/upfiles/2696.gif" width=40 border=0> </P>   <P>312 </P>   <P>312 </P>   <P>  </P> '
insert # select '<P>  </P>   <P>123 </P>   <P>12 </P>   <P>312 </P>   <P>312 </P>  </P>'
go


select replace(substring(news_content,charindex('src="',news_content),charindex('.gif"',news_content) - charindex('src="',news_content)+5),'"','') as src 
from #
where charindex('src="',news_content) > 0 
if object_id('tempdb.dbo.#') is not null drop table #

/*
src      
---------
src=/WebSite/upfiles/2721.gif
src=/WebSite/upfiles/2696.gif

*/