[[pdoResources?
&parents=`7`
&sortby=`{"publishedon":"DESC"}`
&limit=`6`
&tpl=`ListRowTpl`
&includeContent=`1`
&prepareTVs=`1`
&processTVs=`1`
&includeTVs=`article.portal,article.start,article.kategorie,article.picture`
&tvFilters=`article.start==Ja%`
&tpl=`tpl.post-startsite`
&tplFirst=`tpl.post-startsiteFirst`
&tvPrefix=`tv.`
]]<div class="col-md-6">
<div class="post">
<a href="#" class="feature-post-link">
<div class="catagory"><span class="label label-default">[[+tv.article.kategorie]]</span></div>
<div class="post-gradient">
[[ImagePlus?
&tvname=`article.picture`
&type=`tpl`
&docid=`[[+id]]`
&tpl=`tpl.firstIntroPic`
]]
</div>
<div class="post-feature-content">
<span class="post-font-date"><i class="fa fa-calendar" aria-hidden="true"></i> 15.10.2017</span>
<p class="post-feature-font-header">[[+pagetitle]]</p>
</div>
</a>
</div>
</div><img src="[[+source.src:pthumb=`w=320`]]" alt="[[+alt]]" class="img-responsive post-img" />
This question has been answered by optimuscrime. See the first response.

In the template Chunk of the getResources/pdoResources Snippet call you could use the placeholder [[+tv.name_of_your_tv]] if the Output Type of the TV is set to Image+. Without additional changes, the placeholder contains the url to the cropped image.
<img src="
[[ImagePlus?
&tvname=`article.picture`
&type=`thumb`
&docid=`[[+id]]`
&options=`w=320`
]]" class="img-responsive post-img" />