If you want to stretch the images to full width or want the images to touch the edges of mobile screen then you can implement that using this jQuery and css. You can apply this in your wordpress post and pages images too.
Here is the jQuery ( you will need to change the CSS Class in the code)
<script>
jQuery(document).ready(function(){
jQuery(".entry img").each(function(index, element) {
jQuery(element).wrap("<span class='stretch'></span>");
});
});
</script>
Related CSS
@media (min-width: 320px) and (max-width: 767px) {
.stretch {
margin-left: -25px !IMPORTANT;
margin-right: -25px !IMPORTANT;
display:block;
}}