When I was a moderator there was extensive discussion of formalizing guidelines for credit. For various reasons there is no formal policy - it's an "act in good faith" situation. The presumption is that anything available at this site is available for use in whole or part by any member of the community. Some people uploaded work to other sites, some things got lost when the db was hacked a few years ago, some were made as part of a scenario and not uploaded separately, and so on. If not sure who to credit post an image here & someone will probably recognize it. Don't need to ask permission. Just give credit. There are rare exceptions, notably Ares request to ask permission before modifying the terrain graphics he has uploaded. Nothing against using them, just changing them without prior permission.