The short answer is that Fanpop provides an "image credit" field which every user should use to make it clear that no ownership oder rights to the Bilder are implied Von being gepostet here: the Bilder still belong to <these people> (what Du insert in the image credit field).
Do a Suchen of Fanpop (or just participate in a Forum long enough) and Du will quickly find dozens of pages of people discussing how to properly use the image credit field. One fairly straightforward method is described in this article:
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