I regard that sort of question to be a trick question, worthy only of being avoided if possible and given a trick answer, if avoiding it is impossible.
In nearly every case where someone asks for my "honest opinion" of something, I've found that my honest opinion was the last thing they really wanted. Yet, I've never really been able to figure out what they were really asking me for; the best I can do is assume they are asking for some encouragement or flattery, neither of which I'm natural at giving.
Although, I've never tried putting myself in the place of someone who is asking that question, at a time when no answer is expected, until now. Under what circumstances would I ask someone for an honest opinion, and what would I hope to gain from it? I suppose the only situation where I would do that would be when the subject is not directly what I'm interested in. Instead, I would ask a question like that because I want to know what the other person sees, how he/she thinks, what kind of reasoning is being used, and how it all differs from my thought process. And so an example of one of the most honest and direct versions of the "what is your honest opinion..." question would be when someone once asked me, upon hearing some music in a style we didn't agree on, "what do you hear in that kind of music... what are you listening for?"
In short, I suppose that someone asking for my comments on their picture is actually asking me to compare notes with them to figure out if we're seeing the same thing, and trying to figure out what I think about them, and how I react to them. In a way, I suppose it's their way of telling me "you aren't giving me enough feedback about who I am and what I'm supposed to be in relation to you, so I'll try asking for that feedback more directly. I don't want to do it this way, but, really, you keep your thoughts and your relationship with me a big damned mystery, and I can't stand that situation any longer!" And so, in a way, I suppose my first instinct (that they are looking for comfort and reassurance) may be correct, after all? Except that in truth, it was me that has been making them insecure and causing them to ask for that reassurance... I always assume that I'm not important enough to cause people to feel uncomfortable and insecure, and so I assume that a question like that has absolutely nothing to do with me directly, and I assume that they asked for my "honest opinion" because mine was as good as anyone else's, when, in fact, my opinion alone was requested for a very specific reason.
That's curious... it gives me an entirely new way of looking at things that have been completely and frustratingly incomprehensible to me, up till now....