Album art & folder pictures...

I have lots of music folders. Most have the correct album art -- though I don't know where it comes from or how it gets displayed -- but some have pictures that have nothing to do with what's in the folder. I have displayed hidden files and there aren't any hidden graphic files in these folders. Also, I have used the properties dialog to change the folder picture file and even though it appears to work properly, the folder picture does not change.

I've looked around but haven't found anything that explains any of this or gives me some idea of how to force the folders to display the correct album art or picture of my choice.

Can anyone offer any help on this?

Thanks in advance,