From my personal experience in Cardiff, you should include 2-4 photos of you and your husband/wife, any record of your 3 holidays (plane ticket, hotel reservation etc) and some chat history.
I went in on Monday and didn't include any of the above as I thought they weren't needed for extension, but I was briefly interviewed and the caseworker even told me that he needed to seek advice from a senior whether I needed to post the evidence back to show the relationship is genuine. Luckily the casework told me they are happy this time to proceed, but he also told me he needed to write a note on my file and requested that I bring in "evidence" in the next submission.
Interesting - that has not been required for anyone else before. I wonder if you were assessed by someone who didn't really know what they were doing?
When you apply in the UK, your genuine relationship is shown by including 6 documents addressed to both of you at the same address spread evenly over the length of time you have been living together, up to a maximum of 2 years.
So, since you already live together, you don't need to show a genuine relationship through holidays or visits because you see each other every day. And, since you are both present in person, a photo is not usually needed, because the fact that you are standing next to each other is proof you have met in person.
I assume the only reason you might need to show evidence of photos for FLR(M) is if your UK sponsor is not able to be present at the appointment, and evidence of trips/holidays would only be needed if you have not yet been able to live together (i.e. if you are switching from a work or student visa to FLR(M)).