Folder thumbnails display random images

This has been bugging me for a while!

Very often when I open a folder to view its contents and choose a thumbnail display ( as opposed to list, details or tiles ) then the folder icons are emblazoned with .jpg images seemingly chosen at random from the images on my hard drive. ( Actually it might not be entirely random, the images displayed are often those I've viewed or edited recently )

It's a pain in the pudenda because after I have chosen an appropriate image to identify the contents of a folder ( say a picture of the Eiffel Tower to denote that the folder is full of pictures of Paris ) the next time I open My Pictures ( or wherever the folder is stored ) I get a random image ( say, a Peruvian llama )

Doesn't only affect picture folders and it even affects the folders that don't allow customisation ( ie those that have no 'Customise' tab in 'Properties' )

I have tried deleting one image that was appearing frequently from My Pictures but it still appears on the folder.

Am I missing a trick with the "Remember each folder's Settings.." stuff? should I be messing with "thumbs.db" ?

I really don't want to lose the work I've put in choosing pictures for the folders that are correctly adorned so a "Global/Nuclear" option would be a real last resort!

I'm using a hp desktop computer running Windows XP SP2