"Another thing. Permission = Written permission. There is no such thing as someone saying to themselves, 'It's OK for people to download my images.' That's not permission."

Section 106:

** 106. Exclusive rights in copyrighted works**
Subject to sections 107 through 122, the owner of copyright under this title has the exclusive rights to do and to authorize any of the following:

There is no copyright law definition for "authorize" nor for "permission" stipulating that permission must always be "written"; therefore it is up to the discretion of the copyright owner as to what form the authorization takes place. Just because the majority of time permissions are set in written form - usually for contractual purposes as evidence against willful infringement - it does not mean that this is the case 100% of the time.

Besides, if they wrote in a thread that they authorize such use, then it is, in fact, "written permission"; further dismissing your objection. :0P

Also there is a stipulation under Fair Use (Section 107) for "nonprofit educational purposes". Therefore, if someone is using a copyrighted work to educate themselves without profitting from the act, it is viable Fair Use according to the law.