Hi Eugene
I've always used my own all-star logos in my fictional leagues and I've had no issues seeing them being used by OOTP. But what I do is directly save them into my saved game team logos folder. That way your fictional league will see them and not need to go looking in OOTP's general logo folder.
Here is where to save them:
C:\Users\Your Name\Documents\Out of the Park Developments\OOTP Baseball 21\saved_games\Your Fictional League.lg\news\html\images\team_logos
I hope this helps.