You can use a tool like Firebug or Safari’s Web Inspector for these kinds of situations. Once you have it installed, you can simply right-click and select “Inspect Element” (or something like that) and you can see what CSS rules apply to a particular part of a website. You can even do live testing of different CSS rules without fear of breaking anything.
For your second question, you should always make a child theme instead of directly editing the theme’s files. If you don’t make a child theme, and your theme gets updated someday, you’ll lose all the edits you made to the theme. Once you have your child theme, you would copy content-single.php
from the parent theme to your child theme’s folder. Edit that file in whatever editor you prefer and delete this code block:
<div class="blog-image">
<?php
if ( has_post_thumbnail() ) {
$image_src = wp_get_attachment_image_src( get_post_thumbnail_id(),'featured' );
echo '<img alt="post" class="imagerct" src="' . $image_src[0] . '">';
}
?>
</div>