I'd just like to add something...
This is a fantastic feature!
I'm just worried about the implementation of it. Perhaps Artist is actually a special case - as people have mentioned, you might want different artist pictures for different eras. It also depends upon MC being "intelligent" as to what folder is actually Artist level. For example, I have a Sort Artist field. Would MC know to present an artist image when at Sort Artist level, especially if there isn't a field called Sort Artist and the view step is defined as an Expression that uses another field? And what about Composers?
Ignoring Artists for the moment, let's consider artwork generally for any folder level. Such as the clapperboard icon in the DLNA server for the Video level folder.
When you define a View, you define Menu Items, and each Menu Item consists of a sequence of Categories.
For starters, I'd like to see an attribute on the Menu Item where you can define your own icon. Menu Items might be Audio, Video, Images, but there might be sub-items such as Music Videos, Movies, Camcorder Videos for example under the Videos menu entry. It would therefore be nice to be able to specify your own icon for each of these.
Then we come to Categories. These define how your library is to be grouped, sorted and presented to you. Artist is only one of a myriad of fields that you can use to group by. You might like to have a custom icon for each Genre for example. You might have categories of your media items such as Album, 7" Single, 12" Single, Compilation Album, Soundtracks - where it would be nice to depict each folder by a custom icon. TV Series have also been mentioned. So I'd like to see an attribute on each Category where you specify how the image representing that folder-level is to be generated.
It would take the form of a pathname, where you can use the value of a field in order to parameterise the image, e.g. C:\mediafiles\myicons\genres\<%itemvalue%>.jpg, if you have a directory of your genre images, the currently selected genre would be passed as the <%itemvalue%> parameter, e.g. C:\mediafiles\myicons\A-Z\rock.jpg. Perhaps one of your Categories is an A-Z index, you could actually define icons at theis level with large capital letters! e.g. C:\mediafiles\myicons\A-Z\<%itemvalue%>.jpg, where again your <%itemvalue%> placeholder is replaced by the actual value of the folder-level. So if MC is displaying a folder for the F index, this pathname would resolve to C:\mediafiles\myicons\A-Z\f.jpg
If no pathname is specified, or if no file can be found with that name, then MC would fall back to its default algorithm of looking up Artist artwork if it's at artist level, and finally as a last step generating the image from the artwork associated with the first media file found underneath that folder (which equiates to Album level).
Does that make sense?