[ronie]

Actually, copying all the extrafanart images over would work just fine for me...would be happy to do it manually if necessary, although if it could be automated that would obviously be much sweeter.

If I do it manually, where should they all be copied to?

well, that would be difficult as the script uses hashed foldernames,
they're created in ../userdata/Thumbnails/ArtistSlideshow.

Also, before switching over to your awesome script, I was able (in Backrow) to nominate a folder with generic music fanart images for cases when no fanart was found. Would it be feasible for a similar folder to be designated (and manually filled) for the last.fm slideshow when no artist images are found?

i think in the end, in T! i'll try to implemnet this kind of fallback
1) extrafanart slideshow
2) last.fm slideshow
3) default artist fanart image
4) default music backdrop

should be enough i hope. :-)