As I can see the images are in 2 resolutions: smaller 555x417 and bigger about 1280x720 (some are weird like 1279x716 or 1283x722). First thing the renders should be at same resolution for best processing by game that either downscale it or upscale to final resolution and current final resolution for game is 544x416 - the screenshot I posted before - you need to change some game coding first if you want it bigger - I saw some RPGM games that does this, but I'm not expert at this game engine, so I cannot tell how to do this.The preview images are the ones in the game.
During a creation phase, I was afraid that the game would become too big, so I tested by making smaller images... it was a bad idea.
These are probably those CGs you're talking about.
I will do them again in better quality, but it is not my priority at the moment.
The new images are in better quality