Each image is composed of a matrix of 8-bit pixels (for each color) that usually are represented using integer numbers. If you mus use for example qpsk for the OFDM carrier you must split these 8 bit in group of 2 bit , This means that you must extract  4 group od 2 bit from each 8-bit pixel

Ex the number 251= 11111011 will be splitted in

11 11 11 10 11 that are equals to the numbers  3  3  2  3  You must put in the qpsk mapper the numbers 3 3 2 and 3

if you need bpsk you must extract single bit because bpsk allow to sent 1 bit for symbol