Taks this as an example.
im = imread("coins.png");
imshow(im);
if you want to convert the matrix into either row or column vector:
im_row = reshape(im,1,[])
im_col = reshape(im,[],1)
if you just swap the row and column (n x m >> m x n)
im_reshape = reshape(im,size(im,2),size(im,1));
imshow(im_reshape);
Note that the pixels are numbered from top to bottom and from left to right.
