discuz的头像是通过一个php中转重定向的，如果并发量上来后效率较低，可以替换为如下的rewrite规则
RewriteCond /usr/local/bbs/%{REQUEST_FILENAME} !-f
RewriteRule /ucenter/data/avatar/(.+)_small.jpg$ /usr/local/bbs/ucenter/images/noavatar_small.gif [L]
RewriteCond /usr/local/bbs/%{REQUEST_FILENAME} !-f
RewriteRule /ucenter/data/avatar/(.+)_middle.jpg$ /usr/local/bbs/ucenter/images/noavatar_middle.gif [L]
RewriteCond /usr/local/bbs/%{REQUEST_FILENAME} !-f
RewriteRule /ucenter/data/avatar/(.+)_big.jpg$ /usr/local/bbs/ucenter/images/noavatar_big.gif [L]