Problem with thumbnails

if ( function_exists( 'add_theme_support' ) ) {
	add_theme_support( 'post-thumbnails' );
        set_post_thumbnail_size( 150, 150, true );
}

<div class="news_img">
					<?php if ( has_post_thumbnail() ) { the_post_thumbnail( 'single-post', array('class' => 'news_img') ); } ?>
					</div>

div.news_img {

	width: 150px;
	height: 150px;
	float: left;

	padding: 10px;

}

div.news_img img {	

	border: solid black thin;	

	display: block;
	margin: 0 auto;
}

The problem is that I can’t make any thumbnail during writing the post because when I choose: Use as featured image i get the answer: Could not set that as the thumbnail image. Try a different attachment.

What should I do? Where is the problem?