After perusing through my AD&D 2nd Ed Player's Handbook (black cover), I noticed on the inside of the back cover were pictures of various products from the different campaign settings available at the time. Under the Planescape banner was a picture of 2604 - Well of Worlds with totally different cover art! After diving down teh Googles, I also found that there were different cover art for the following early Planescape products:
2601 - Eternal Boundary
2602 - Planescape Monstrous Compendium Appendix
2604 - Well of Worlds
2605 - In the Abyss
2600 - Planescape Campaign Setting
2603 - Planes of Chaos
Most notably, all of these products had a slightly different Planescape logo as well.
I know that Dana Knutson was the conceptual artist for the Planescape line of products at TSR, but unfortunately his website, danaknutson.com, went offline between August 2018 and December 2018. I believe there is a good chance that Robh Ruppel may have been the original artist of these works. They were definitely not done by Tony Di'Terlizzi.
Does anybody know how I can track down higher quality versions of these art pieces, or how I can get my hands on a Planescape Sketchbook?
To my knowledge, there are not any to be found on the internet. If I find any of the low-res scans again, I will post them below.