If the compliment sounds genuine and impulsive, I appreciate it--even if it sounds sort of funny because the person didn't take the time to think it through.
I often think complimentary things about other people, but there's some filter on my tongue that requires me to think through every darn thing before saying it. By the time it gets past the filter, I've thought of half a dozen reasons why I shouldn't say it, and half a dozen why I should, and I'm frustrated because I wish I could have just said it when it crossed my mind.
When someone blurts out, "What a beautiful blouse!" or "Wow. You have amazing eyes," and it's obvious that they were just saying it because it's what they think and they aren't expecting any particular response from me, then it makes me happy. On the other hand, a compliment from a stranger followed by, "Want to go get coffee?" irritates me for some reason. As though the fact that I have a nice shirt or nice eyes obviously indicates I'd be an enjoyable coffee-companion. I dislike the implication that someone is judging me based on my appearance (even if they're right and it's an effective way to judge people...). On the other hand, if I were in a check out line at a bookstore, and got to chatting with someone about books, and we started sharing thoughts, and then he said, "You have some great ideas. Want to get a cup of coffee and continue the discussion?" I'd probably be flattered.
I don't appreciate flattery with sexual undertones from a stranger. (example: "You look amazing in those jeans.") But it could be very nice from a friend.
As for posting a picture on a forum, I generally don't post one until I'm confortable in the community. There's always the option not to post a pic, but in real life there's hardly the option of being invisible. If I join a forum and post a picture, that's a good indication that I don't mind remarks about my appearance.