Oh, neat. These are the four Grand Masters from the Trading Card GB games. I always really liked how each opponent had their own unique portrait in that game, so it's cool to see some fanart of some of them (even if it is the big name guys and not the lesser trainers.)

Nicovideo:
http://seiga.nicovideo.jp/seiga/im4764064

Pixiv:
https://www.pixiv.net/member_illust.php?mode=medium&illust_id=43158714