Your code looks OK and it should work just fine. But there are some things you can (and you should) fix.
1. You don’t use absolute URL for your fallback image
You pass /wp/wp-content/themes/klicknet-theme/images/testbild.png as src of your image. It would be much better and more secure if you’d use WP functions in there. For example like so:
<img src="https://wordpress.stackexchange.com/questions/307591/<?php bloginfo("template_url'); ?>/images/testbild.png">
2. You don’t escape the title properly
In your fallback image you use the_title() in title attribute. But you don’t escape it as an attribute. If the title contains " character, it will break your HTML. Another problem is that the title can contain HTML tags, and they will be printed in your attribute.
If you want to use title as attribute, you should use the_title_attribute function instead. So the fixed version of that line can look something like this:
<img src="https://wordpress.stackexchange.com/questions/307591/<?php bloginfo("template_url'); ?>/images/testbild.png" alt="testbild" width="334" height="259" title="<?php the_title_attribute( array( 'before' => 'Bild: ', 'after' => '' ) ); ?>">