It's not wrong to like a female picture.
It is wrong to assume giving her a compliment will make her day better. In fact, you will most certainly make her day worse if you are sending these sorts of comments on italki. You will be identifying yourself as the kind of man that women want to get away from.
Yes it's wrong. Don't do it. How would your partner feel?
And, whether you are in a relationship or not, it would still be wrong. It's amazing that you think your "compliment" will "make her day better". Maybe she will hate what you did, and hate being judged on her looks. Suppose the female were your sister ; would you be happy with guys doing that ?