Ok let me officially answer this as an Art Team Leader. Sorry for the long read, it is a subject we are working on.
1: The rules for this is being discussed and a new tool is being created where art submissions can be managed by the AT as well as by those who claim the art. So for instance (from my understanding) the art that is made by an artist can be uploaded and then they can manage it to be linked to others whom purchase. The details on this tool are pending though.
2: Officially all customs and images submitted are officially owned by SWCombine, this is why we have them not only edited but also have an official mandate to replace all non member original content from SWCombine official rules images.
3: Customs are both owned by the player who submits (pays the CPs) as well as SWCombine but not outside of SWCombine, meaning that the Art Team will remove images that an artist can prove they made within SWCombine but outside SWCombine use is not managed or controlled.
4: Art that is non-edited other than a background but is associated with a non combiner Artist, can be requested by the original artist to be removed from SWCombine as well. Edited content is covered under fair use (20%+ surface edit supported) but only for use with SWCombine.
5: Generally speaking if you hire an artist, you buy a copy of the artist's work for personal use only and if you are contracting them to make the work for public or multi use then it must be stated in contract of sale and agreed upon. It is covered under something called fair use.
6. Art that is submitted for a custom, applied, and then the object it is tied to changes hands or if the artist changes their mind... will not be removed. So long as it is personally applied and not the art itself being sold then the artist cannot fairly request a removal as it was for personal use and then exchanged hands. Though I do request that if you plan on using a piece of art from an artist for mass production that you inform them to prevent drama and wading into the muddy water of nit picking at details.
6: Original artwork that is player made and given to SWCombine for official imagery is considered "Owned" for use within SWCombine and is compensated with a form of currency we commonly call CP's.
Ok, while the rules on this are going to be sorted out by the Art Team in coming months as we get the new tool so we can enforce/manage properly as is needed, then we will as a matter of course also update and publish a policy on this. This issue is very cloudy but overall the way things are going to be managed is basically to what we think is fair. If an artist comes to the Art Team and contests art used for anything other than personal use then we will investigate and if the artist can prove they did the art we will enforce it.
For now I am volunteering to manage any Custom image issue that arises. I want to work with all players to come to a fair deal for everyone. Generally speaking, CPs are spent on submitting art and those are only gained through doing work for SWCombine in some form and thus will be honored both that you use them for personal use as well as making sure others do not profit off your work by using a personal copy for public use (thus allowing them to gain CP's).