Good question. The backgrounds are from other creators. The preview images actually use RPg Maker MV RTP backgrounds that were "pixel-ized" using a program called "pixatool" which is available on itch. there are a handful of background creators out there (I love Tokiwa Graphics backgrounds) and if you use consistent palette and pixel-ization rate it matches really well. I should probably build a little tutorial...
Viewing post in Tyler Warren RPG Battlers - Pixel-Style 1 comments
Ah, yes they are RM only. depending what you're looking for you can modify any battlebacks basically the same way. I've used some of Joel's backgrounds in different projects as well https://joelsteudler.itch.io/overworld-battlebacks Looking through the RM storefront battlebacks is definitely a thin offering and some are RM only. Maybe I need to make some of my own :)