How to tell somebody that they had really a good pose in a picture?
Can I say it like this?
What a pose
Or is it inappropriate?
What a pose!
To answer your question, yes, that three-word phrase can be used as an exclamation, and I think it would generally be regarded as a compliment. However, a little more context would help to make your intentions clear.
Adding an adjective might be a good idea:
What a great pose!
because without it, the phrase could also mean one of these:
As for it being "inappropriate," that depends on the situation. If I was looking at a coworker’s Facebook page, and she was posing rather flirtatiously, it might not be a good idea to say "What a pose!" in the workplace. But assuming you're not going to cross any boundaries of professionalism, there's nothing wrong with the remark.
In fact, the construct isn't all that not uncommon; we can use "What an X!" as a shortened way to express our approval about something. For example:
What a night!
can mean, "What a great night we had last night," or, "What a dreadful night we had last night." (Context usually makes it clear. If I'm talking about an lovely evening of dancing, I'm saying it was great. If I'm talking about staying in the emergency room until 3AM because my daughter was injured, I'm talking about something different.) Similarly, I can say,
What a car!
when I see a car driving down the street. If the car is a sleek Lamborghini, most would interpret my meaning to be, "What a sweet car!" If it's a jalopy, however, my meaning would be, "What a piece of junk!"
If you want to say that someone looks good in a particular photo, say it just like that:
You look really good in that photo.
Alternatively, you might concentrate on a particularly nice feature of the person:
You have such a lovely smile in that photo.
Another more general compliment to pay someone regarding how good they look in photos is to say:
You're really photogenic.
It sounds a little unnatural to refer to the specific pose that a person makes. Normally we use the word "pose" to mean "adopt a position for a photo to be taken". As such, the word is good for describing the act of assembling for a photo, but not so great for commenting on the look of the subjects of a photo.
We say that people "strike" a pose when they are being photographed. To compliment on the pose pictured, expressing a surprise (that what your "what a pose!" essentially does) is not enough. A compliment ought to contain something positive.
So, add positive evaluation to it somehow, for instance, "Your pose here appears really nice" or "You struck a neat pose" or "You look good in this photo, especially I like your pose".