Ah, it's one of those issues:
"Timeless" vs "Timeless (2016)"
"The Goldbergs" vs "The Goldbergs (2013)"
Then you are correct. If you run the "Get Movie & TV Info" on one episode and select the correct TVDB ID, then the background image gathering would start to work.
Note that if your background images are changing, your metadata for the program episode is also incorrect.
The fundamental issue is whether the Series name in your EPG data matches your Series name in MC, and hence MC retains the correct TVDB ID for the series. The [TheTVDB Series ID] tag is relational, so setting it for one episode of a series sets it for all episodes... as long as the [Series] tag matches for all episodes.
There was an issue a while back where MC seemed to lose its assignment of TheTVDB ID when new episodes of a series with this issue came along, but I thought that was fixed. I assume you are on a recent version of MC?
If you select all episodes of the Series, make sure they all have the same Series name and that it matches your EPG data, and run "Get Movie & TV Info", selecting the correct Series, MC should remember that (and fix all metadata) and work in future.
You could set the Series name to, for example, "Timeless (2016)" in the recording rules. But you would also need to change the Series name for all existing episodes so that they match. That should work, but it shouldn't be necessary.
Anyway, search for discussions on "fixing" the "Doctor Who" series in MC, particularly recent discussions. You have the same problem as described in those threads.