I’ve read the imgur terms of service and regarding copyright it says under the heading of “Stuff not to do”,
“if someone else might own the copyright to it, don’t upload it.” http://imgur.com/tos
There’s more about copyright further down under the “Details” heading.
“Intellectual Property
By uploading a file or other content or by making a comment, you represent and warrant to us that (1) doing so does not violate or infringe anyone else’s rights; and (2) you created the file or other content you are uploading, or otherwise have sufficient intellectual property rights to upload the material consistent with these terms.”