Hi-Res Pokémon! - Rescuing official Pokémon Art!さんのイラストまとめ

January, February, March and April from the 1998 CoroCoro Pikachu calendar by Kagemaru Himeno.

Late 1996 or early 1997, rare nurses by Ken Sugimori, heavily enhanced from low-quality assets found on the original Game Freak website. Mind these have been heavily reworked, hence the full-res original might look somewhat different if found!

1997, Illustrations by Kagemaru Himeno for the '97 Pokémon Fanbook, you can read a translation of the captions on tumblr, link in the thread!

2003, Red and Leaf promo art for Pokémon Fire Red and Leaf Green by Ken Sugimori, these two are also available in stupid resolution XD links in thread.

Bill, Prof. Elm, Prof. Oak and Gold's mother from the 1999 GS set by Ken Sugimori

Ken Sugimori, posted on Game Freak’s official website around January 1999. Original title みんな走れ！(Everybody run!). Enhanced and upscaled from the original.