Index Artwork

[NightSprite]

I have a related query, although more from a beginner's standpoint. My music library is stored on a Windows PC and streamed to a Linn DSM using one of the 2 Linnn apps (whichever happens to feel like working) on an iphone. (Is there a better control app for iOS?).
I tend to use folder view and most of the artwork displays without problem. However some do not, even though there is a jpg file supplied with the download. It appears entirely arbitary which display and which do not.
Some folders do not have a jpg file and yet the artwork displays anyway (I assume it is embeddded in some way)
I have tried making my own images and putting them into folders which do not have artwork but none of those has ever worked. I would love to choose a specific image for the 'Jazz' folder and then another image for the artist folder and then the specific album artwork for the albums in the folder. My understanding is that that should be possible. I can't find any clue as to why some work and some don't; the one's that don't work, don't work irrespective of which view I use. I have tried 'indexArtwork=auto' and '=all' but it doesn't seem to make any difference.
Any advice would be appreciated.