If the element you're analyzing does not have any margin, border or whatsoever defined you won't be able to return it. At tops you'll be able to get 'auto' which is normally the default.
From your example I can see that you have margT as variable. Not sure if're trying to get margin-top. If that's the case you should be using .css('margin-top').
You're also trying to get the stylization from 'img' which will result (if you have more than one) in an array.
What you should do is use the .each() jquery method.
For example:
jQuery('img').each(function() {
// get margin top
var margT = jQuery(this).css('margin-top');
// Do something with margT
});