To celebrate the 23rd anniversary of Chrono Trigger's original North American release, I played through the Steam version and got all the achievements.
Unfortunately, the images aren't simply sitting around in the game's installation folder as I had hoped, and I don't know how to extract them. I took screenshots at the best resolution I could, but that's only 1680x1050. They came out blurry-looking, despite being PNGs.
I've also included images from the Steam trading cards and profile backgrounds for the game. They're JPEGs, but they're bigger (1920x1080) and clearer than my screenshots. Unfortunately, the full-body character portraits from the game's bonus features weren't available.
Hopefully these will be helpful.