What about "hình"? Is that also a "photo" or a "photograph"? My family is from the south and we use "hình". Is "ảnh" a word northerners typically use and "hình" a word southerners use?
Yes, it can be also used. I think you are right that "hình" is in Southern dialect.
Why is there no classifier for photo?
I'm pretty sure 'ảnh' is itself a classifier... God, I'm fairly lost with this one.
I was wondering this same question. The wiktionary says that the classifer can be either one of these:
"tấm" (large flat things) or
"bức" (indicates walls, barriers, handwritten letters, drawings, paintings, photographs, sculptures or statues)