If you're 99% sure it's using NTSC artwork on both versions, then I'm 100% sure. I've seen idiots attempt to come at me before when mentioning NTSC/PAL issues trying to say there were separate PAL versions that would run and look fine, that the internet is just populated with NTSC images (AHAHHAHAHAHAHHAHAHAHHAH!----- breath----AHAHHAHAHAHAHAHHAHAHAHAHhA ----- AHHAHAHAHHAHAHHA)
Overwhelmed with NTSC images? lol. I don't believe this game is rare like you do, what a collector can and cannot find online in physical form (or digital with the prevalence of PAL, not NTSC images online) says nothing about its availability at the time. #6 best game of 1992, with a nice hefty price tag I might add, they wren't afraid that people wouldn't buy it at 70 bucks in 1992 money. It was out there.
Anyway, until one of those people come to me with said perfect looking and running PAL game ported from NTSC, I'm telling you, I ain't ever seen an example of NTSC stuff converted to PAL when it comes to artwork. Music? Sometimes they make that play at the correct speed across versions. Three Stooges was the same way, there was a region lock on it, but it still only looks right in NTSC.
Pinball Dreams... Now there's one I need to take a look at. That was a 320x256 PAL game. It came to America, but there was a problem... They couldn't make a lazy port and just crop out the top, because that's where the scoreboard was.... So they did screw with the graphics for the NTSC version of Pinball Dreams... I'm still not sure if they just moved the scoreboard or if they stretched the graphics to look correct in NTSC 320x200... but I've long been curious about that. There's an actual reason there, they couldn't just throw it over, they had to mess with it. Otherwise, if they don't? They won't. They didn't.