what is the most efficient way to loop through picture pixles

i've 2 images that i need to compare of 1280x800 each and i'm too worried about the efficiency as ill have to do the same operations that include looping each second

i can think of so many ways to loop through a Bitmap object pixels but i don't know which would be more efficient, for now i am using simple for loop but it uses too much memory and processing which i could not afford at this point

some tweaks here and there and all it does is less memory for more processing or the other way around

any tips, information or experience is highly appreciated, i'm also open to use external libraries if they have much better efficiency

thanks in advance and have a wonderful day