use the following search parameters to narrow your results:

subreddit:sub

find submissions in "sub"

author:username

find submissions by "username"

site:example.com

find submissions from "example.com"

url:text

search for "text" in url

selftext:text

search for "text" in self post contents

self:yes (or self:no)

include (or exclude) self posts

e.g. subreddit:pics site:imgur.com dog