I prefer free-to-use assets over AI generated ones. I have no problems with you keeping your costs down. Backgrounds are not as important as having a good story to drive the VN
Viewing post in Itch.io Situation, Pogress and AI Art comments
I've read my fair share of visual novels and I'm also in agreeance with this comment. However I must also say that AI can give you a better background to go off. but it's more likely to be incorrect or has parts that don't make sense.. and if you do end up using some AI backgrounds make sure it is mentioned on your page because some people get crappy about it.
Basically as long as the free-use images fit the Vibe you're after.
As Witch stated quite rightly it is not as important as having a good story.