Featured image and window resizing

I made a child theme and set this in my css:

.tc-rectangular-thumb img.attachment-tc-thumb {
  width: 100%;
  left: 0px;
}

The objective is to have featured image to always fit in the parent container. What happens is this: while the page is loading the image is fitted as aspected. When the page finish loading (and javascripts start to do their job) the image is resized (bigger than the container) and cropped. How can I avoid this behavior?
If this is a javascript function that makes this where is it hooked? thanks in advance.