You can’t use the post content to add an image outside of the call to the_content().
If you want to look into implementing Post Thumbnails (i.e. Featured Images) into your Theme.
Full implementation is outside the scope of a simple question-and-answer; however, in the end, your markup will change from this:
<h1 class="title"><a href="https://wordpress.stackexchange.com/questions/87953/<?php the_permalink() ?>"><?php the_title(); ?></a></h1>
<?php the_content(); ?>
…to something like this:
<?php if ( has_post_thumbnail() ) { the_post_thumbnail(); } ?>
<h1 class="title"><a href="https://wordpress.stackexchange.com/questions/87953/<?php the_permalink() ?>"><?php the_title(); ?></a></h1>
<?php the_content(); ?>