It should be on by default. But the setting is in biography.ini directly above where you would have switched on the cover setting: Image [Artist] Auto-Fetch=1. Did you click the panel to switch it to artist view? Music files must valid tags.
Artist images saved to the "Image [Artist] Folder Location" can be displayed with other artwork and playlist viewers. One image will be picked for each artist. For this you need to enter a path in preferences\display\album art. Choose the artist tab.
For the default save location, example paths you need to put in preferences>display>album art and search pattern for ARTIST are:
Standard installC:\Users\YOUR USERNAME\AppData\Roaming\foobar2000\yttm\art_img\$replace($cut(%artist%,1),.,_)\$if($stricmp($cut(%artist%,1),.),_$substr(%artist%,2,$len(%artist%)),%artist%)\*
REMEMBER to enter YOUR USERNAME
Portable install
J:\foobar2000\yttm\art_img\$replace($cut(%artist%,1),.,_)\$if($stricmp($cut(%artist%,1),.),_$substr(%artist%,2,$len(%artist%)),%artist%)\*
REMEMBER to set the correct drive letter and path to foobar2000
Edit the above base paths if necessary according to your set-up