I think that the original question is too simple to answer definitively. It depends on many factors. Some that immediately come to mind are the relationship between my partner and the other person, my relationship between me and the other person, our relationship (me and partner), the context of the picture, previous discussions and agreements of boundaries, history of the relationship, etc. For example, it's one thing for them to sit on the lap of a family member that I have a good relationship with for a family photo in which many people are sitting on laps, and it's another thing for them to have cheated previously be sitting on the lap of the person that keeps telling her to break up with me for them after I've expressed concern over their relationship while taken in a gondola when I wasn't even aware they were together that night.
I think it's best to talk about it with your partner. If you don't like how you feel about it during the discussion or find that you are having to convince yourself or suppress your emotions, then you're probably not okay with it. That would take further introspection on your part. I've been told to always trust my intuition, which means that if something doesn't feel right or feels odd/weird, then there's likely a valid reason for that feeling that isn't just me making it up to start problems. Another approach is to notice how your partner responds to your concern. Do (a) they respond how you would respond if they brought it up to you and you wanted to make sure they were happy with the discussion, or (b) does it become antagonistic (you vs. them)? The latter would be a sign that it's not healthy for you.
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"Am I wrong?" - Walter Sobchak