Thanks Peter for helping.
Yes, docker on linux, upgraded once from 1.8.3 to 1.8.7. The problem of the thumbnails was already there.
Last year we was using xibo 1.7.9 non-docker, on the same centos machine. We wipeout everything, and reinstalled centos7 and xibo-1.8.3 docker, so it’s really weird that these pictures are still there.
I tried just right now, I cleared all the caches with ccleaner. I relogged and the wrong thumbnail is still there.
MySQL [cms]> SELECT * from module WHERE `Module` = 'notificationview';
Empty set (0.00 sec)
MySQL [cms]> SELECT * from module;
+----------+---------------+----------------+---------+----------------+-------------------------------------------------------------------------+------------------------+---------------+---------------------------+----------------+------------+-----------+------------------------------------------------------+------------+---------------------------+-----------------+-------------+
| ModuleID | Module | Name | Enabled | RegionSpecific | Description | ImageUri | SchemaVersion | ValidExtensions | PreviewEnabled | assignable | render_as | settings | viewPath | class | defaultDuration | installName |
+----------+---------------+----------------+---------+----------------+-------------------------------------------------------------------------+------------------------+---------------+---------------------------+----------------+------------+-----------+------------------------------------------------------+------------+---------------------------+-----------------+-------------+
| 1 | Image | Image | 1 | 0 | Images. PNG, JPG, BMP, GIF | forms/image.gif | 1 | jpg,jpeg,png,bmp,gif | 1 | 1 | NULL | NULL | ../modules | Xibo\Widget\Image | 10 | NULL |
| 2 | Video | Video | 1 | 0 | Videos - support varies depending on the client hardware you are using. | forms/video.gif | 1 | wmv,avi,mpg,mpeg,webm,mp4 | 0 | 1 | NULL | NULL | ../modules | Xibo\Widget\Video | 0 | NULL |
| 3 | Flash | Flash | 1 | 0 | Flash | forms/flash.gif | 1 | swf | 1 | 1 | NULL | NULL | ../modules | Xibo\Widget\Flash | 10 | NULL |
| 4 | PowerPoint | PowerPoint | 1 | 0 | Powerpoint. PPT, PPS | forms/powerpoint.gif | 1 | ppt,pps,pptx | 1 | 1 | NULL | NULL | ../modules | Xibo\Widget\PowerPoint | 10 | NULL |
| 5 | Webpage | Webpage | 1 | 1 | Webpages. | forms/webpage.gif | 1 | NULL | 1 | 1 | NULL | NULL | ../modules | Xibo\Widget\WebPage | 60 | NULL |
| 6 | Ticker | Ticker | 1 | 1 | RSS Ticker. | forms/ticker.gif | 1 | NULL | 1 | 1 | NULL | [] | ../modules | Xibo\Widget\Ticker | 5 | NULL |
| 7 | Text | Text | 1 | 1 | Text. With Directional Controls. | forms/text.gif | 1 | NULL | 1 | 1 | NULL | NULL | ../modules | Xibo\Widget\Text | 5 | NULL |
| 8 | Embedded | Embedded | 1 | 1 | Embedded HTML | forms/webpage.gif | 1 | NULL | 1 | 1 | NULL | NULL | ../modules | Xibo\Widget\Embedded | 60 | NULL |
| 11 | datasetview | Data Set | 1 | 1 | A view on a DataSet | forms/datasetview.gif | 1 | NULL | 1 | 1 | NULL | NULL | ../modules | Xibo\Widget\DataSetView | 60 | NULL |
| 12 | shellcommand | Shell Command | 1 | 1 | Execute a shell command on the client | forms/shellcommand.gif | 1 | NULL | 1 | 1 | NULL | NULL | ../modules | Xibo\Widget\ShellCommand | 3 | NULL |
| 13 | localvideo | Local Video | 1 | 1 | Play a video locally stored on the client | forms/video.gif | 1 | NULL | 0 | 1 | NULL | NULL | ../modules | Xibo\Widget\LocalVideo | 60 | NULL |
| 14 | genericfile | Generic File | 1 | 0 | A generic file to be stored in the library | forms/library.gif | 1 | apk,ipk,js,html,htm | 0 | 0 | NULL | NULL | ../modules | Xibo\Widget\GenericFile | 10 | NULL |
| 15 | clock | Clock | 1 | 1 | | forms/library.gif | 1 | NULL | 1 | 1 | html | [] | ../modules | Xibo\Widget\Clock | 5 | NULL |
| 16 | font | Font | 1 | 0 | A font to use in other Modules | forms/library.gif | 1 | ttf,otf,eot,svg,woff | 0 | 0 | NULL | NULL | ../modules | Xibo\Widget\Font | 10 | NULL |
| 17 | audio | Audio | 1 | 0 | Audio - support varies depending on the client hardware | forms/video.gif | 1 | mp3,wav | 0 | 1 | NULL | NULL | ../modules | Xibo\Widget\Audio | 0 | NULL |
| 18 | pdf | PDF | 1 | 0 | PDF document viewer | forms/pdf.gif | 1 | pdf | 1 | 1 | html | NULL | ../modules | Xibo\Widget\Pdf | 60 | NULL |
| 19 | googletraffic | Google Traffic | 1 | 1 | Google Traffic Map | forms/library.gif | 1 | NULL | 1 | 1 | html | {"apiKey":"1234567812345678"} | ../modules | Xibo\Widget\GoogleTraffic | 20 | NULL |
| 20 | forecastio | Weather | 1 | 1 | Weather Powered by DarkSky | forms/library.gif | 1 | NULL | 1 | 1 | html | [] | ../modules | Xibo\Widget\ForecastIo | 60 | NULL |
| 21 | hls | HLS | 1 | 1 | HLS Video Stream | forms/library.gif | 1 | NULL | 1 | 1 | html | [] | ../modules | Xibo\Widget\Hls | 60 | NULL |
+----------+---------------+----------------+---------+----------------+-------------------------------------------------------------------------+------------------------+---------------+---------------------------+----------------+------------+-----------+------------------------------------------------------+------------+---------------------------+-----------------+-------------+
19 rows in set (0.00 sec)
I think is not there. Anyway, everything is working correctly. Only the notification widget vanished, and this thumbnail “bug”… i used quotation marks cause maybe it’s a personal bug 
thanks for this amazing assistance,
Francesco