While almost all of the time Tom would have only been brought in to illustrate the box art or promotional art, in an interview with Arcade Attack he shares that he actually had a hand in the creation of the Sparkster character (hence this page of concept art):

"R.K.A. was the exception to the rule where they brought me in early on top create the Rocket Knight character. I remember Konami wanted to get a game out there to compete with Sonic the Hedgehog."

https://www.arcadeattack.co.uk/tom-dubois-cover-artist/