So, firstly does SongKong find the artist artwork, you can check this by looking at the report and going to Browse By Artist/Album. If a picture is displayed for an artist then SongKogn has found one, if it displays a folder icon it has not.
Secondly, MinimServer does not currently support artist images (although it is planned, and is high priority) so you cannot currently see artist artwork when you browse by artist in your control point. But if you browse by folder you can see artist artwork added by SongKong but only if you have enabled Save album artwork to artist folders in filesystem on the Artwork tab
Hopefully, that makes things clearer.