Add image only in first post

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The error you are getting is quite specific and tells you exactly what is wrong. Also debug errors are off topic here.

Your real issue here is adding an image to the first post only, which you can accomplish with the build-in loop counter $current_post that starts at 0, so the first post will be 0, not 1.

Inside your loop, you can do the following 

if  ( $wp_query->current_post == 0 ) {
    // Add your picture for first post only here
}

EDIT

Your code in your edit should be:

<?php if ( $wp_query->current_post == 0 ) { ?>

    <div class="col-md-6" style="float: right">
        <img src ="https://wordpress.stackexchange.com/questions/188615/<?php echo get_stylesheet_directory_uri(); ?>/images/wolf.png">
    </div>
<?php } ?>

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