Copyright can be a complicated issue so in general it is best to err on the side of caution. As it mentions on the avatar upload part of your profile, "if you are thinking of using an avatar which you have not personally created FROM SCRATCH, firstly CHECK whether you have permission to use it (unless the creator has clearly stated that an image is Public Domain, copyright exists on all creative works, no matter whether there's a copyright notice visible or not). Remember, copyright is law, and if you break someones copyright you are breaking the law".
In some instances it may not be absolutely clear. This is where I prefer to err on the side of caution and say if you don't know for sure that usage permission exists (which has been stated somewhere) it's best not to use it.
Sorry if this seems harsh but I intend to keep the board going on a permanent basis, so need to make sure that things stay within certain rules.
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