Changing Matrix Elements Automatically

I have a large matrix representing an image, 4000 columns, 2000 rows. I would like to change the values of some of the matrix elements and these elements are defined in a pair of vectors. Trying to automate the process because of the extreme size and complexity. I have attached a small example of what I am doing (and doing wrong) in which I have set up a FOR loop. This loop changes all of the elements, not just the ones I want. Help? See attached example.