I just try to install the featured image widget in the sidebar. As I didn't find the category "featured images" I created this category and added an image to it. But it does not appear in the sidebar ... the headline "featured images" is there, but no image below it. Could anyone give me a hint what I missed to do.

Great. Thanks for the fast help again, Leo

You mean the order in which they show? It might be dependent on which order you add them, but I don't know. Generally 6-12 featured images should show...easily seen by the viewer without the order being a huge problem.

I'll be tightening things up soon so they show better as well.

OOOooooooOo 
You wanna message microstockman and see if he will surrender some of his CSS to you

Okay, I got a css-code:

           .symbiostock-featured {
        display: block;
    margin-left: auto;
    margin-right: auto;
    text-align: center;
        padding-left: 10px

but it has no effect. I fear I have done some basic mistake somewhere. For example it has no effect if i create a text-widget and insert it in home-page (below content) or home page bottom row.
Leo, you offered some days ago to have a look at my site, when i create an admin account for you. It would be great if you are still willing to have a look and see, whether there is a fundamental wrong setting.

Yes, i have the closing bracket in my styles.css
I just missed to copy it here.

.symbiostock-featured.search-result {
    float: left;
    height: 170px;
    margin: 10px;
    padding: 0;
    text-align: center;
    width: 160px;
}
This should work. Let me know if it does anything scary, bad, or weird.

Great! That works ... strange enough: it works after copying styles.css in the main theme not in the child theme.
But it has still no effect if I create a text-widget and insert it in home-page (below content) or home page bottom row.
I have set a static page: Parent is Sample Page, Template is Default Template ... anything wrong with that perhaps?

Okay, thanks Leo