There are multiple methods that would work well, but I'll explain how I did it. I just used our actual names in the photos (first name only for the two of us). But on the top corner of each page of photos (I put several photos in a document I printed out on computer paper), I also had an address-sized sticker label that said "Sponsored Person: [my full name], Sponsor: [my husband's full name], and the form [IMM 5490, Supplemental Information] to which the photos correspond, so that probably covers it if there was any confusion as to who is the applicant and who is the sponsor (or if the paper accidentally gets separated from the rest of our application and needs to be re-matched with it).
Underneath each photo I made a caption. Each one started with the exact date (which I tracked down using our calendars and the info embedded in the digital photo) and stated the occasion (visit with family, birthday, trip to wherever) and listed each person in the photo, their relation to us (mother/friend/best friend/friend of family/etc) and the location of where the photo was taken.
Using your example as inspiration, I would have said something like:
25 August 2005: Sandra and John at Sandra's apartment in New York
or
25 August 2005: Sandra and John with Jane Doe (Sandra's mother), when Sandra introduced John to her mom for the first time over cupcakes at Magnolia Bakery in New York City.
My thought was that I would rather have a little too much detail than leave whoever was reviewing our application having unanswered questions about what this photo says about our relationship. I think the main thing is to remember the photos are to show your relationship covers a range of time, that you have spent various occasions together (normal days, birthdays, holidays, trips, etc), and that other people have met your spouse (if that is the case).
