Hi Mahesh,
thank you for information, it seems, that the previous html I was using didn’t work with current theme version. The previous html I used was:
<a href="http://www.lepszapracalepszezycie.pl/seria-szef/"><img src="http://www.lepszapracalepszezycie.pl/wp-content/uploads/2016/11/Seria-szef.png" alt="Image Widget" /></a>
Now I changed it to:
<a href="http://www.lepszapracalepszezycie.pl/seria-szef/"><img src=http://www.lepszapracalepszezycie.pl/wp-content/uploads/2016/11/Seria-szef.png></a>
and it works.
However I still have problem with thumnails in the wordpress popular posts plugin. I have just enabled them again and still only image icon is visible. I would appreciate your help on this.