一丁 is a cut of tofu or konyaku, so why is it also used for photos? I always imagined it as a Polaroid, flat, like もう一枚 but can't find out the original reason もう一丁 was used for it.
2 Answers
Its use is not limited to photos. もう一丁 is an idiomatic phrase which just means "one more try", "give it another shot".
丁 is also used as a counter for dishes (of food), and today もう一丁 is typically heard as a vigorous call in some restaurants (「牛丼、並一丁！」「ラーメン大盛り一丁！ もう一丁！」）. I think you can just memorize it as it is.
Photographers may say もう一丁 to their subjects, but they never use 丁 to actually count the number of photos or trials (e.g. [*]写真が5丁, [*]あと3丁撮ってみましょう).
Formal and Informal Speech
「[丁]{ちょう}」, formally, is used like you said -- a counter for tofu, konnyaku, etc.
In informal speech (mostly among male speakers), however, it can be used to count many different inanimate objects. Saying 「[写真]{しゃしん}もう[一丁]{いっちょう}」 to mean "one more pic" is no problem.
I am sure that is not something they would actively teach in Japanese-as-a-foreign-language but among native speakers, it is just pretty normal.