"When someone gets permission from an artist applies ONLY TO THE PERSON WHO WROTE HIM. Not to the managers of your groups, not to your sister. If you have a graphics design site that have 5 other designs as managers, too – THEY EACH MUST ASK FOR PERMISSION."
As you can see, permission for use must be specific – can the person requesting permission actually alter the image? Are they allowed to create things for others or only for themselves? Do they have permission to make sigs/snags/tags or just to display a few pieces of work?
Also, permission letters are good only for the person who wrote to the artist – they can not give permission to anybody else! Could there be exceptions? Of course there could, but the requestor would have to be very clear that they're asking for permission for their management team/graphic makers/membership to make graphics for themselves &/or for others.
The unfortunate part is that you can never be sure what the person writing to the artist actually requested (usually, only the letter from the artist (or parts of it) are posted for members to see). Even worse, you can't be sure the permissions are actually legit. So, you must take these "permissions" in conjunction with "red flags"/"green lights."
I, myself, am very wary of people using artists work for making things like sigs/snags/tags for other people to download (and even more leery of "blanket permissions" – meaning allowing any manager/graphic maker/member to use the images). It has been my experience that more artists grant permission for personal use web sets (and, perhaps, for link-ware), than for other types of graphics.
The reason, I believe, is because it's far easier to link a web set to an artists site than it is to expect it for sigs & snags. Again, this is just my opinion – there certainly are artists who are more lenient (or, perhaps less net savvy) and it's always good to check the artist's TOU yourself and to check periodically that terms haven't been changed.
In summary: If a group has no "red flags" and has some "green lights" they're permissions are probably legit; be wary of "blanket permissions;" if you are at all suspicious, check out the artists terms yourself; remember that website hosts may not allow links to groups/sites with adult content.
And thus endeth this round of the Safe Graphics School.
The self-quiz is linked below – good luck!