If i understand you correctly you want the folder.jpg displayed in the group header of the ELPlaylist and as well in the popup window instead of the artist picture?
Then it can't be fixed with extended variables but with the setting in ELPlaylist directly.
Go to:
Right-click in the playlist --> Settings --> script --> Popup(Background), scroll down to this entry:
////// Artist Picture //////
$if($findfile($get(artist.pic)),
$imageabs(4,4,$sub(%el_height%,8),$sub(%el_height%,8),$get(artist.pic),wc)
,
$if(%length%,
$font(Segoe UI,7,)
$drawstring(- Image not available -,3,3,$sub(%el_height%,8),$sub(%el_height%,8),192-192-192,hcenter vcenter)
,
$drawimage(4,4,$sub(%el_height%,8),$sub(%el_height%,8),$if2(%darkone_radio_pic%,%foobar_path%DarkOne\Images\Radio.png))
)
)
and change it to:
////// Artist Picture //////
$puts(no.cover,$if2(%darkone_no_cover%,%foobar_path%DarkOne\Images\NoCover.png))
$puts(cd.pic,$if2(%darkone_cd_pic%,%foobar_path%DarkOne\Images\AudioCD.png))
$puts(radio.pic,$if2(%darkone_radio_pic%,%foobar_path%DarkOne\Images\Radio.png))
$if2($imageabs(4,4,$sub(%el_height%,8),$sub(%el_height%,8),%path%,artreader)
,
$imageabs(4,4,$sub(%el_height%,8),$sub(%el_height%,8),$if(%length%,$if($stricmp(%codec%,cdda),$get(cd.pic),$get(no.cover)),$get(radio.pic)))
)