Your code produces a CHAR vector (a "string" is a different class). Now you explain, that there is an error, when you "try to open the string". But what does this mean? Where do you try this in which way? What do you want to see?
A hint: You can replace:
a = reshape(Image.', 1, );
But there is no need to do this. Simply use:
Image = imread('calle1.png');
allOneString = sprintf('%d,', Image.');