Change the search icon in twenty fourteen

This is more of a simple CSS question than actually related to WordPress.

However, you need to change your style.css. As TwentyFourteen does not use images, but pseudo css-classes, you need to change this as well:

.search-toggle {
    background: #fff url('http://www.citizen-science.at/wordpress/wp-content/themes/cs/img/search-icon.png') no-repeat;
    background-size: 30px;
    background-position: 10px;
}
.search-toggle:before {
    content: "";
}

By the way, your background image is huge. Try providing a smaller one.