@texnixe @jimbobrjames @bvdputte - Thanks for helping out, but I don’t get some things.
It’s the first time, I’m stuck with Kirby.
First of all, I set my template.php back to the default. When I now take a look at my RSS-Feed, I got eight articles.
RSS-Feed without publishing-date:
The RSS-Feed got no date. It’s not showing, when the article was written. When I take a look at that code (link above), I’ll find:
<lastBuildDate><?php echo date('r', $modified) ?></lastBuildDate>
When I’m opening up an article in my browser and take a look at my source-code there is a date:
<header class="article-header">
<h1>Das ist nur ein Test</h1>
<div class="intro text"> Dienstag, 17. Juli 2018</div>
<hr />
</header>
So, a date is given, but there is now date in that RSS-Feed. 
Cover-Image:
According to @jimbobrjames I’ve to add code to the description. Okay, now I’ve done that:
<description>
<?php if($image = $item->coverimage()->toFile()): ?>
<figure>
<a href="<?php echo $image->url() ?>">
<img src="<?php echo $image->url() ?>">
</a>
</figure>
<?php endif ?>
<![CDATA[<?php echo $item->{$textfield}()->kirbytext() ?>]]></description>
The feed is empty. Same, if I add the code in front of the </description>.
All images:
Than I’ve tried @texnixe suggestion. Now I can see all images in my RSS-Feed - great.
Funny thing: When there is a Coverimage and I’m using the same image in my article, only one of them shows up. Is that by accident?
Think of my fictional travel-blog. I got a nice picture of Sagrada Família in Barcelona and I’m using that image as coverimage. Later in the article I’m explaining something for that temple, and will show the image again. How would I do that?
@jimbobrjames : You’re suggesting a “custom snippet”. I’m new to Kirby and haven’t read about custom snippets. What is the best way for newbies?