How can I get image locations in a list of entries using the Element API plugin?
This should work for the images and you can easily adapt to do your categories &amp; kachelgroesse if needed (not tested)...

'api/looks/list' =&gt; [
    'elementType' =&gt; 'Entry',
    'criteria' =&gt; ['section' =&gt; 'looks'],
    'transformer' =&gt; function(EntryModel $entry) {
        $images = [];
        foreach ($entry-&gt;featuredImage as $image) {
        $images[] = $image-&gt;url;
        }
        return [
        'title' =&gt; $entry-&gt;title,
        'id' =&gt; $entry-&gt;id,
        'url' =&gt; $entry-&gt;url,
        'jsonUrl' =&gt; UrlHelper::getUrl("looks/{$entry-&gt;id}.json"),
        'kachelgroesse' =&gt; $entry-&gt;kachelgroesse,
        'category' =&gt; $entry-&gt;category,
        'image' =&gt; $images
        ];
    },
],