I'm doing another slut run with CC Mod right now and so far it's been mostly stable. But I had one error message like that, about a missing png file. I just 'fixed' it by going into the folder mentioned in the error message, copying a file that has a similar name as the one missing into the same folder and then renaming the copy to the name in the message. But I kept the file ending and didn't change that to png. Not sure why it says png, maybe the game unpacks these files?
Anyway, that's a quick and dirty way to get past these error messages and keep playing.
But as zoomies mentioned your mod version probably needs updating, because this shouldn't occur directly after the start of the game.
I Tried it with both "Karryn's.Prison.1.0.5.J.2.FULL" and "Karryns_Prison_v1.0.5j_FULL" versions. The MOD version I used was the master branch of the gitgud as well as the dev branch. Both with the same error message when starting up. Looking into the mentioned folder I can see that all of the files are there, but not as png files. I also tried your trick with the renaming, alas that didn't help either (even when I saved it as pngs).
The only part that confuses me is that the MOD doesn't say it's version in the file name. I Just downloaded the newest branch (5 months old) hoping it would be the correct one.
I also attached a screenshot of how i renamed it. Did you do it like this?
Thank you for your quick answers by the way, really apreciate it. Now that the game is finally completed I really want to try it out.
EDIT: Nevermind, actually changing the file endings did help and it seems to be working now (turns out I have to edit all the files in that folder). I'm just slightly spooked that I might encounter error messages. Welp, yolo i guess.