If anybody has additional information, I'd appreciate it. I haven't seen season 2 of Torchwood or most of season 4 of DW, but I'm not scared of spoilers, so if there's anything mentioned in canon about her or about the time he was married that I've missed I'd appreciate it. Info below the cut!
So, according to Wikipedia, sepia-toned photographs were first used in the 1880s- which tells us that this was after Jack joined Torchwood. That gives us an absolute earliest date. Now, looking at the bride's dress (I'm going to call her B, for bride, after this), it seemed very much Edwardian rather than Victorian to me. A white dress is correct, Victoria made them popular when she got married in 1840- though generally, even in Edwardian times, it was only the rich who could afford to get married in white. Of course, with the sepia tone, she could be in light gold and we couldn't tell. The skirt, however, falls pretty straight and the bust and waist shapes are natural, so that says Edwardian to me.
So to narrow down the times a little, I went wedding-dress browsing. Now, here are some dresses from 1910. The top one has a pretty natural waistline, and the ones below that are empire-waisted are bridesmaid dresses. Also, look at how similar the gathering at the waist is compared to B's dress. The bouquets are pretty similar, too.
If you go back to 1904, however, there are more differences- the bouquets are much bigger, the women all seem to have shoulderpads, and there's a LOT of fancywork all over the dresses, whereas B seems to just have some work at the collar (though that could just be fabric texture, I guess, but I don't think so.) Also, every female member of both 1904 wedding parties are wearing very long necklaces, statement pieces. B's necklace is quite short, and has no pendant. And if you jump to 1917, the dresses all get shorter, and while you can't see the bottom of B's dress in Jack's photo, you can't see a hem and from proportions it seems to go down at least a little further than her ankles. Also the shape of the skirts change, and it's wartime, so nobody's spending as much money on these things anymore.
So I'm guessing about 1910 or thereabouts. The dress is nice, and from what I remember from my 1941 copy of Emily Post's ettiquette and my Dorothy Sayers novels set in the thirties, it was a very set thing that the bride or the bride's family paid for the dress. So I'm guessing B's family had a fair bit of money- not rich, perhaps, but comfortable. Or else she was the only daughter or something. The difference in men's fashions is always a lot harder for me to see, so I can't really take a guess at Jack's suit, except to comment that he does look good in it. :)