How to enable thumbnails inside all posts

Open ../post-templates/content-single.php and remove these lines:

<?php if( $show_thumbnail && ac_check_paged() ) { ?>

<?php } ?>

Also in style.css find:

.single-template-1 .featured-image-wrap {
  width: 50%;
  margin: 0 0 35px 60px;
  float: right;
  -webkit-box-shadow: -8px 8px 0 #e1e1e1;
  -moz-box-shadow: -8px 8px 0 #e1e1e1;
  box-shadow: -8px 8px 0 #e1e1e1;
}

Change it with:

.single-template-1 .featured-image-wrap {
  width: 100%;
  margin: 0 0 35px 0px;
  float: right;
  -webkit-box-shadow: 0 8px 0 #e1e1e1;
  -moz-box-shadow: 0 8px 0 #e1e1e1;
  box-shadow: 0 8px 0 #e1e1e1;
}

Also add this:
.single-content.featured-image:before { display:none !important; }

I would recommend you use a child theme.

On the main page it’s a little tricky. All thumbnail sizes were calculated so that they look good on all devices. You need to know some CSS to adjust them for every screen resolution.

I recommend you use http://jobs.wordpress.net/