I think I'm missing something as I've never been married myself, but surely a wedding should be about celebrating your love and commitment to your future husband/wife with all the people that you love, the "coming together" of two families and friendship groups and sharing a special day.
If your friend cares more about pretty photos than all this, then it bodes well for her marriage. I'd rather have good memories of a day than good photos of a day. Basically if she would rather have a "perfect" photo to look at in the future, instead of looking back in the future and remember having all her friends with her enjoying themselves, she is a very shallow person.
If she is willing to treat 'Sophie' in such an awful way because she doesn't want her disability 'ruining' the photos', whereas for me not having my friends by my side as bridesmaids to share and celebrate this special day with me would sooner ruin my experience of the day than an "imperfect" photo she probably won't even look at that often in ten years time, but if she is willing to be so horrid to this friend, what's to say she won't treat you similarly one day. She's obviously very callous and cares little for her "friends".
In any case it is unlikely the disability will 'ruin' any photos. Not everyone is as shallow and discriminatory as her, most people will see the photos, which are usually edited by the photographers anyway, and not really care that one of the bridesmaids has a visible disability. It is only her who would particularly take issue with this.
People saying it is her right to choose who she wants as her bridesmaids, this is true and nobody is suggesting you force her to ask Sophie to be a bridesmaid, but it is your right too to choose not to associate yourself with such a vain person, who will go on to be a terrible friend to you too