use the following search parameters to narrow your results:
subreddit:subredditfind submissions in "subreddit"author:usernamefind submissions by "username"site:example.comfind submissions from "example.com"url:textsearch for "text" in urlselftext:textsearch for "text" in self post contentsself:yes (or self:no)include (or exclude) self postsnsfw:yes (or nsfw:no)include (or exclude) results marked as NSFW
e.g. subreddit:aww site:imgur.com dog
see the search faq for details.