Is it ok to ask for opinions about a specific UI example? or do questions have to stay objective and theoretical?
It would be OK to upload an image of a UI, but I would still ask a very specific question about it. Not just 'what do you think?' but 'what do you think about the position of this button?'.
There's a million different UI's, and if everyone uploads theirs and asks for opinions, it's going to be a huge mess. Asking specific questions is going to narrow this down, and opens possibilities for reuse. Similar UI elements might be found in other applications as well, and questions about them will be of use to others.