The term "distance" implies, that there are two different objects. So you can calculate trhe distance from one object to another only. Currently you have a "568x1 image", what is a really unusual format for an image, which are usually rectangular. Then you need another 568 x 1 vector to calculate the distance to:
img = rand(568, 1); % Test data!
img2 = rand(568, 1); % Missing in your question
dist = sqrt(sum((img - img2) .^ 2));
