I was uploading some images I had just created to imgur earlier today and noticed that chrome couldn't access my Pictures folder. Windows tells me access was denied. Firefox didn't have an issue though

I went to it through windows explorer and it worked fine, and looked at the security tab under properties and noticed that there were four unknown accounts, one of which has full control privileges.

I looked at my other folders in the same drive and none of them had these unknown accounts. It was only that specific Pictures folder, and all of its subfolders.

What are these unknown accounts and what could it mean? Should I be worried that someone may have compromised the system (well, I should probably be worried about that all the time I guess)

I read on microsoft support forums that it may be the result of a previously deleted account, but there has only been one account on this computer for months and no user account management has been performed for awhile.