So may I ask you: your problem is solved or not?
I think your problem is about the name of the album art.
Just name your album art cover as folder.jpg, cover.jpg, front.jpg, back.jpg, cd*.jpg, or %album%.jpg, etc. whatever you like. Folder.jpg is recommended.
Suppose you use folder.jpg as the cover's name, and keep the album art cover in the same album folder where the audio file is:
1)If you are using Default UI, like sindred said, this shouldn't be a problem.
2)If you are using Columns UI (latest version), there are several panels can display the album art; depends on what components you have installed.
-2a) Artwork view (Columns UI built-in): In the Preferences -> Columns UI-> Artwork repository -> Front scripts:
CODE
$replace(%path%,%filename_ext%,folder.*)
( * means various format, support jpg, png, bmp etc.)
-2b) Album Art: In the Source tab, just use the default setting.