I’ve been going through Base Set and I’ve so far matched up all but 5 of Ken Sugimori’s 32 Base Set artworks with stock images:
- 17/102 Beedrill (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006003&i=2
- 20/102 Electabuzz (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=8&fid=99007088&i=87
- 22/102 Pidgeotto (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006031&i=30
- 23/102 Arcanine (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006029&i=28
- 26/102 Dratini (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006200&i=199
- 27/102 Farfetch’d (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=10&fid=99009191&i=190
- 28/102 Growlithe (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006033&i=32
- 30/102 Ivysaur (Ken Sugimori) - ?
- 31/102 Jynx (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=8&fid=99007004&i=3
- 32/102 Kadabra (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=8&fid=99007108&i=107
- 34/102 Machoke (Ken Sugimori) - ?
- 36/102 Magmar (Ken Sugimori) - ?
- 38/102 Poliwhirl (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=5&fid=99004157&i=156
- 40/102 Raticate (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=10&fid=99009063&i=62
- 41/102 Seel (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=5&fid=99004145&i=144
- 42/102 Wartortle (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006144&i=143
- 45/102 Caterpie (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006109&i=108
- 49/102 Drowzee (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=8&fid=99007043&i=42
- 54/102 Metapod (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006072&i=71
- 55/102 Nidoran♂ (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006037&i=36
- 56/102 Onix (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006167&i=166
- 57/102 Pidgey (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006006&i=5
- 59/102 Poliwag (Ken Sugimori) - ?
- 60/102 Ponyta (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006012&i=11
- 62/102 Sandshrew (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=9&fid=99008176&i=175
- 68/102 Vulpix (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=8&fid=99007009&i=8
- 73/102 Imposter Professor Oak (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=9&fid=99008070&i=69
- 75/102 Lass (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=10&fid=99009194&i=193
- 76/102 Pokémon Breeder (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=7&fid=99006025&i=24
- 77/102 Pokémon Trader (Ken Sugimori) - ?
- 88/102 Professor Oak (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=9&fid=99008151&i=150
- 91/102 Bill (Ken Sugimori) - www.sozaijiten.com/cdrom/imagedetail.asp?pid=9&fid=99008004&i=3
Most of these are just the raw images with some kind of filter applied, but some are flipped (i.e. Kadabra) and others are recoloured (i.e. Metapod).
All of these are from only 5 stock image volumes:
- Vol.4 〈炎・火花・水編〉 (www.sozaijiten.com/cdrom/sozaijiten.asp?vol=4)
- Vol.6 〈四季・自然編〉 (www.sozaijiten.com/cdrom/sozaijiten.asp?vol=6)
- Vol.7 〈光・クリスタル編〉 (www.sozaijiten.com/cdrom/sozaijiten.asp?vol=7)
- Vol.8 〈壁・土編〉 (www.sozaijiten.com/cdrom/sozaijiten.asp?vol=8)
- Vol.9 〈絵の具・パステル編〉 (www.sozaijiten.com/cdrom/sozaijiten.asp?vol=9)
A lot of people complain about 5ban but at least those don’t use stock images for backgrounds. 
It does make me wonder what involvement Ken Sugimori had in the original Pokémon sets, as it wouldn’t surprise me if some intern just took his Pokémon drawings and stuck them on fitting stock backgrounds.