According the Rule 2 of the Forum Guidelines
, if you possess verifiable consent of the owner of copyrighted content, you may post the allowed content (within the limits of the content rules).
Which means that you may use copyrighted content, including pictures, only if you possess consent of the owner of it.
Recently, we've received the Copyright Infringement DMCA Takedown Notice towards the forum avatar of one of this forum's users.
An artist said the image in question was taken from their website and used without permission. We removed it immediately.
Usually, users are free to use pics as avatars unless the artists say otherwise.
So, folks, make sure the image you use as an avatar here can be used that way. Pay attention also to pictures you use for your characters in-game and then post on this forum for other forum members - you can do that only if you credit the artist and that artist is not against such use.