Not sure about specific sites, but I know a lot of folks here find smilies on the internet, upload them to BYC (using the "Uploads" option on the blue menu line) and copy them into their posts.
Can you tell that I know just enough about smilies to be dangerous?
Yep, you can right click on any smiley you see in a post, do a copy and paste to another post. To save it for later you have to save it to your computer then either upload or copy and past every time you use it. Uploading it to here is more useful IMO.