"귀엽다" is used when you talk to yourself. Of course, it is a base form of "귀엽다." There are tones of way you can conjugate the word, "귀엽다."
For example, your friend shows a cute picture of his dog to you. And you might want to say to yourself by saying "어~~~ 귀엽다!" But it's like an exclamatory phrase. You don't expect the other person to respond to your speech. Just because you just talk to yourself...but we are human beings...if someone talks to themselves when someone is near you, usually they react. I think so. I read the phrase. Please, check it.
"귀여워요" is used when you communicate with someone. In the same situation like I described it above, you say "귀여워요" in order to communicate with the one who showed the picture. When you say this phrase, you might want to look at the person who showed the pic. It might sound like:
Please, leave a comment when you have a further question.