[polskieradio] Add thumbnails.
diff --git a/youtube_dl/extractor/polskieradio.py b/youtube_dl/extractor/polskieradio.py
index d3bebaea32f..9e7eab12edc 100644
--- a/youtube_dl/extractor/polskieradio.py
+++ b/youtube_dl/extractor/polskieradio.py
@@ -33,6 +33,7 @@ class PolskieRadioIE(InfoExtractor):
                 'timestamp': 1456594200,
                 'upload_date': '20160227',
                 'duration': 2364,
+                'thumbnail': 're:^https?://static.prsa.pl/images/.*\.jpg$'
             },
         }],
     }, {
@@ -68,6 +69,8 @@ def _real_extract(self, url):
             r'(?s)<span[^>]+id="datetime2"[^>]*>(.+?)</span>',
             webpage, 'timestamp', fatal=False))
 
+        thumbnail_url = self._og_search_thumbnail(webpage)
+
         entries = []
 
         media_urls = set()
@@ -87,6 +90,7 @@ def _real_extract(self, url):
                 'duration': int_or_none(media.get('length')),
                 'vcodec': 'none' if media.get('provider') == 'audio' else None,
                 'timestamp': timestamp,
+                'thumbnail': thumbnail_url
             })
 
         title = self._og_search_title(webpage).strip()

### What is the purpose of your _pull request_?
- [x] Bug fix
- [ ] New extractor
- [ ] New feature

---
### Description of your _pull request_ and other information

This is a simple change to extract thumbnails from Polskie Radio auditions.
