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?
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?