Just tried the Refresh Artwork for some of the items in the library like the sort by age rating and the first time I tried on one of the sections it actually changed the actual picture. After the first time and for all other sections and sorts now it only changes the color tint overlay, not the actual picture.
Just to add
The background images for the buttons (such as on the genres bars or release decade filters) appear to be drawn from either the TMDB-sourced top-rated title in each category, or its most popular one. It might change if you move titles in or out of your library but it won’t ever change if the lists’ contents don’t change (or new content isn’t rated higher than the existing title whose fanart is displayed).
I don’t know how Infuse chooses among multiple items with the same rating / popularity but I know I’ve never been able to change the title chosen to represent each item … and it is one of my biggest pet peeves about Infuse’s UI.