Calculating Unicity Distance

Question

Hi I am trying to encrypt a grey-scale image with pixel values ranging from 0-255. I want to determine the unicity distance assuming that the stream cipher generates bytes of key and each byte is added to the pixel in my image using modulo 256 addition.

Please clarify since I am using a stream cipher should I consider my plain text as binary digits of 1 and 0 or can I consider 256 pixel values to be my plain text.