It's immediately interesting to me that, although you have made clear that you don't like having your photograph taken, she does not respect your boundaries. It makes me wonder: what are her reasons for doing this?
Have a conversation with her. Ask her if she has noticed that her photographing you makes you uncomfortable and explain why you feel this way as far as you are able.
Mum, have you noticed how I always try to avoid photographs? Well, it's because [reasons].
She may then try to explain why she wants to take photographs of you. If she does, do listen. If she doesn't, ask, listen and genuinely consider her feelings and how they relate to your own. If she invalidates your feelings by saying something like 'don't be silly' or 'don't make a fuss' or makes arbitrary excuses, emphasize that 'no, I really mean it - I feel really uncomfortable and I want to have a real heart-to-heart about this'. It can help to set a tone for this conversation by asking in earnest for her to explain her reasons and listening carefully, commenting in summary to indicate that you are also taking her point of view on board:
I see that photographs of me are important to you because [reasons].
If her reasons make you feel differently,* then suggest a compromise. For example, if you feel up to it, you could set a photo limit (e.g. you will pose for one group photograph).
If you absolutely do not want to be photographed for whatever reason, this is absolutely fine and you should feel justified in asserting this. It would be helpful to reiterate that you have heard your mother's point-of-view, but draw the line in unambiguous terms.
I know that it's important to you that you have photographs of me because [reasons], but I really feel uncomfortable and I'm asking you to respect how I feel.
It may also be worthwhile suggesting an alternative, or spontaneously providing one. For example, if your mother wants to have photographs of you so that she can show her friends how you're doing, suggest instead that she show them photos of where you live/study/work/do hobbies. Sending her some photos of these things now and then would be nice. Maybe she wants to have memories of you, in which case you could gift her something you've made (e.g. a scrapbook-style collage). One last catch-all suggestion is to send photos to her of something you are nurturing (e.g. a pet, a plant, a collection). These alternatives might help her meet her emotional needs, hopefully lessening the burden being imposed on you.
As an example: As a teenager, I used to have a strong dislike of photographs because of how I felt about my looks. My mother would insist that I be photographed, which really made me feel uncomfortable. I told my mother that it made me uncomfortable, explaining that I felt ugly and did not want to be photographed. Of course, as a mother, she explained that it upset her to hear that I felt that way about myself. I think my making myself vulnerable encouraged her to reciprocate, and she explained that the reason she wanted to have these photographs was because she was terrified of losing her memory. We reached a compromise, where I agreed to be in group photographs only. It was a valuable conversation which was good for our relationship.