Series poster issues

I know I've missed an answer somewhere in my searching... if fact I thought I had an answer, but it didn't work for me... So here's the issue.
For series I have a folder named 'ShowTitle', and inside I have 'ShowTitle S01E01.mp4', 'ShowTitle S01E02.mkv', etc. If I place posters as 'ShowTitle S01E01.jpg', 'ShowTitle S01E02.jpg', etc. then each show has the image shown in place of the automatically generated thumbnail. I only want one jpg for ALL shows in this folder though, so I don't have to copy and rename the jpg each time I add a new episode. From other posts I thought it sounded like having a single jpg of any name should work. I've tried having 'ShowTitle.jpg', poster.jpg, dvdcover.jpg and other but it didn't seem to work and would always show auto thumbnail instead. Any ideas on how I can have just one jpg for the folder to display the image for ALL episodes in that folder?

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I don't remember how it works in this case, but do you have metadata updates on? When you add folder.jpg, force refresh, will it change?

I do have meta update on though I should note that my serviio box is an offline server with LAN but no internet connection so the only data it gets, I put there. As for force update, if I add a 'title S01E01.jpg' matching the movie file name exactly, then forcing update does cause the image to change. Any other named jpg doesn't as far as I can tell, leaving only the auto generated thumbnail.

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So if you remove all the other .jpg images and only leave folder.jpg, does that work?

This did fix my issue. I thought the poster jpg name didn't matter as long as there was only a single jpg in the folder. By calling it folder.jpg it is now using this image as thumbnail for all files in this folder. Thanks for the help!