So I've come to learn that something + だね means something along the lines of "Is it __?" with an expected yes answer. I received this kind of comment as a response to a photo I posted on the internet. What are some common/friendly ways to respond to this?
I think ね is sort of rhetorical, and I'd only answer it if I was talking to someone in person. Even then, the answer would probably be something perfunctory like そうだ, just to feign attentiveness ;)
I could be wrong; I learned this from Tae Kim, so you can check out that reference and see if it helps. Also, the example sentences at Tangorin and Jim Breen's WWWJDIC come off as making it sound rhetorical/for flavor, in the same category as よ and ざ.
I think it would depend on the context. For example, if someone commented to my photo:
これ、プードルだね。It's a poodle, right?
インドカレーだね。It's Indian curry, isn't it?
あ、ディズニーランド(に行ったん)だね！Ah you've been to Disneyland?
then I would respond:
うん！/はい！/ そうよ。/ そうだよ。Yes.
そうよ、かわいいでしょ。Yes, isn't it cute?
はい、おいしかったです！Yes, it was yummy!
うん。楽しかったよ。Yes, I had so much fun. etc...
If someone commented:
オシャレ/ステキだね。It's so fashionable/cool.
ハンサム/イケメン/美人だね。You look so handsome/pretty.
かわいいプードルだね。Your poodle is so cute!
かっこいい車だね。What a nice car!
(to the food I prepared) おいしそうなカレーだね。Your curry looks yummy.
then I would respond like:
そう？ありがとう！/ ほんと？ありがとう！^^ Really? Thank you!
If someone said:
そっくりだね。You two look so alike.
珍しい花だね。It's an exotic flower, isn't it.
変な動物だね。It's a strange animal, isn't it.
then I would say
そうでしょ。/ そうね。(feminine) / そうだね。(masculine) Yes, I know.
You're right about the expected "yes" answer. 「～だね。」 is a rhetorical question, and a full answer is not expected. ね is a sentence ending particle that changes a statement into a rhetorical question. だ is the casual copula used with nouns or な-adjectives.
The asker is usually mentioning something they think (subjectively) and they expect you to agree with their thoughts. Answering with「うん、そう」 or even simply 「そう」 would work as a general answer.
*As mentioned in the comments below, 「そう」is unnatural as a response to a compliment. In that case you can use 「そうだね」（そうですね）to agree without sounding too arrogant.
It is also common to respond by repeating the statement asked. For example:
Ａさん：今日は寒いね。 (Today is cold, isn't it?)
Ｂさん：うん、寒い。(Yes, it is cold.)
Unfortunately I don't have enough rep to comment, so I attempted a full answer. This answer comes from my limited experience, so please correct me if I am wrong.