Could you please help me with the following too?
I got a picture from friend, looked at it and just saw a few people rock climbing (people, I didn't seem to know). Quite a bit later, when I took a closer look, I recognized that one of the people is this friend. How could I say in a short sentence, that it was a moment of surprise when I saw him in the picture, but that it was a nice surprise. Would one of these sentence work in this context? It was a nice surprise to discover you in the picture.
It was a nice surprise when I discovered you in the picture. (This is probably more about the moment when I recognized him?)
I also like your suggestion " It was an unexpected pleasure ..." but that probably wouldn't work in this context, or does it?
Well, he did send you the photo himself, so it is natural enough that he should be in it.
If I've understood you correctly, you were surprised only because you didn't recognise him immediately and later realised that one of the apparent strangers in the photo was your friend. But he wouldn't know that.
(Or was there some other reason for your surprise? Because he's not the type who engages in activities like rock-climbing?)
I'd probably say "I liked the photo you sent" or "You look good in that photo" or "Nice to see you enjoying yourself". Something like that - I suppose you get the picture.