2.1) 36
2.2) 7111
2.3) they are not high or low, they can be any value in a range, thus it is not digital, the pixel values can be digital, as they are a single representable value, which can be written in binary.
2.4)i) 20
2.4)ii) 26
2.5) it is not worth it, as it makes the file larger