Board Thread:Support Requests - Designing Your Wiki/@comment-5466786-20130819031851/@comment-5285329-20130819214333

You can change the welcome message from:

MediaWiki:Welcome-message-user if the wiki you administrate has user talks for new users. And for anons

MediaWiki:Welcome-message-anon

And you can change it form:

MediaWiki:Welcome-message-wall-user if the wiki you adminstrate has message walls. And for anons

MediaWiki:Welcome-message-wall-anon

Hope this is what you're looking for!