Be sure to read https://yatse.tv/wiki/debug-yatse-kodi-remote to provide necessary logs and information.
Issue description:
I’ve got several movies that do not have a poster.jog, but only a thumb.jpg (created and used by kodi), but yatse doesn’t display it!?
How am I able to have a picture for those movies in yatse, too?
Logs:
Screenshots:
Additional information:
Yatse displays what Kodi sends via it’s API it does not care at all about naming.
I could have helped you more, unfortunately you seemed to have on purpose ignored the template 
Please provide the asked logs so I can tell more.
I’m not suspecting a bug, but didn’t just find the answer to whether thumbs are equally handled as posters when no poster is found.
Kodi creates a thumb preview if no poster is found and displays that as the cover (poster). As my home movies have no posters, but only thumbs, kodi reads those thumbs and displays them. Yatse only displays posters and fanart, but nothing (only yatse’s default image when no artwork exists), if only thumbs are available.
So kodi either doesn’t send them or yatse doesn’t ask them, that’s my understanding so far.
What is yatse explicitly asking kodi for as a preview?
Btw, if I cache the artwork from ytase settings, the preview count it is caching is far less than my movie count in total, although all movies have either poster or thumb available
Yatse ask for all arts and support thumbs.
So please provide the logs, asking theoretical questions is just making both of us loose time for nothing
I’ve send a debug log per mail
Steps I took:
Add NFOs for the movies
Start with a fresh kodi
Add source and scrape local information only
Let kodi create thumbs
Export DB (thumbs are created fine)
Add kodi to yatse
Update movies (movies are shown without thumb)
Cache art from yatse settings
Still no art within yatse
Movie folders look like:
filename-thumb.jpg
filename.avi
filename.nfo
This example is with kodi Leia, but my main use for the actual Media is 17.6
I lack Kodi logs but
2018-12-07 13:25:59.972 Error/ImageRequestCacheStreamFetcher: i.e@48: getImageInputStream [http://192.168.5.211:8080/image/image%3A%2F%2Fvideo%40%252fmedia%252f8TB%252fBilder%252fCanon%25REMOVEDi%2F] failed with http: 404
2018-12-07 13:25:59.973 Verbose/KodiLogger: ← [976] File not foundFile not found
Kodi returns URLs for the images that are invalid, depending on your Kodi 18 version this can be caused by some “security” they added.
Aha…
Some security? 
I stumbled upon these lines in log:
{“art”:{“thumb”:“image://video@%2fmedia%2f8TB%2fBilder%2fCanon%2fMovies%2fBulli%2fBulli%20Abschied%201%2fBulli%20Abschied%201%20JPEG-480p%20PCM-1ch.avi/”}
where it should be:… PCM-1ch-thumb.jpg
`
Not it should not, once you have scanned the media, this is the internal url for autogenerated thumb.
To have the exported one, you need to remove the source once the thumbs is created, then cleanup Kodi, then re add the source and then it will use the generated thumb saved.
Anyway from the logs you gave, the issue is on Kodi side.
Kodi security : fix: Cve 2017 5982 by wsnipex · Pull Request #14501 · xbmc/xbmc · GitHub
Fix that should allow those urls back : Fix remote access to images accidentally blocked by Cve 2017 5982 patch by DaveTBlake · Pull Request #14783 · xbmc/xbmc · GitHub
Could also be No web interface images with libmicrohttpd-0.9.61 · Issue #14991 · xbmc/xbmc · GitHub
14:16:41.544 T:140507508832000 DEBUG: CAnnouncementManager - Announcement: OnScanStarted from xbmc
14:16:41.544 T:140507508832000 DEBUG: GOT ANNOUNCEMENT, type: 16, from xbmc, message OnScanStarted
14:16:41.549 T:140506797647616 DEBUG: VideoInfoScanner: Scanning dir ‘/media/8TB/Bilder/Canon/Movies/Bulli/’ as not in the database
14:16:41.557 T:140506797647616 DEBUG: VideoInfoScanner: No (new) information was found in dir /media/8TB/Bilder/Canon/Movies/Bulli/
14:16:41.561 T:140506797647616 DEBUG: VideoInfoScanner: Scanning dir ‘/media/8TB/Bilder/Canon/Movies/Bulli/Bulli Abschied 1/’ as not in the database
14:16:41.567 T:140506797647616 DEBUG: VideoInfoScanner: Found matching full NFO file: /media/8TB/Bilder/Canon/Movies/Bulli/Bulli Abschied 1/Bulli Abschied 1 JPEG-480p PCM-1ch.nfo
14:16:41.579 T:140506797647616 DEBUG: Caching image ‘/media/8TB/Bilder/Canon/Movies/Bulli/Bulli Abschied 1/Bulli Abschied 1 JPEG-480p PCM-1ch-thumb.jpg’ to ‘f/fd95db8d.jpg’:
14:16:41.579 T:140506797647616 DEBUG: cached image ‘special://masterprofile/Thumbnails/f/fd95db8d.jpg’ size 640x480
14:16:41.619 T:140506797647616 DEBUG: VideoInfoScanner: Adding new item to movies:/media/8TB/Bilder/Canon/Movies/Bulli/Bulli Abschied 1/Bulli Abschied 1 JPEG-480p PCM-1ch.avi
14:16:41.639 T:140507508832000 DEBUG: CAnnouncementManager - Announcement: OnUpdate from xbmc
14:17:19.784 T:140502531827456 DEBUG: CWebServer[8080]: request received for /jsonrpc
14:17:19.905 T:140502531827456 DEBUG: CWebServer[8080]: request received for /image/image%3A%2F%2F%252fmedia%252f8TB%252fBilder%252fCanon%252fMovies%252fBulli%252fBulli%2520Abschied%25201%252fBulli%2520Abschied%25201%2520JPEG-480p%2520PCM-1ch-thu
mb.jpg%2F
14:17:19.906 T:140502531827456 DEBUG: CUtil::GetMatchingSource: no matching source found for [/net/atom/media/8TB/Bilder/Canon/Movies/Bulli/Bulli Abschied 1/Bulli Abschied 1 JPEG-480p PCM-1ch-thumb.jpg]
14:17:19.908 T:140503077091072 DEBUG: Previous line repeats 5 times.
You lack the most important part (The Kodi version) but this is the “Security” thing
the above was leia.
I sent kodi and yatse logs (17.6) per mail.
this is the “Security” thing 
Hm. So, maybe it’s a “security thing” with v18-rc2, but there even are no thumbs for v17.6
Just to be clear I do Kodi stuff since 9 years 
This is not maybe your previous logs are clear and if it’s RC2 you really should report that to them or it will be in final release.
Your last logs are incomplete, you need to kill : restart Yatse if you change Kodi version without changing host.
And needed logs are when browsing the movie part not the fill cache one.
I re-checked with 17.6 and now thumbs are shown!? I’ll go and re-add my media to the Kodi I’m using yatse for (LE 8.2.5, RPI3) and report back.
The solution from this thread is:
For kodi 17.6
If adding a source to kodi with local information onky, that has no thumb jpgs, prior to adding the source, kodi creates the thumbs and is able to export them to filesystem.
Yatse is able to see and display those thumbs only after removing and re-adding the source, after the thumbs are read from filesystem
For kodi 18
We’ll see, if the bug report is helpful
Closing as it seems they do not want to fix the global issue. Adding new sources will be the workaround 