Is there a plugin to watermark all the users uploaded images?
The default wpForo post attachments are stored in WordPress media library and are being passed through all WordPress functions. If you install some WordPress image watermarking plugin it may also affect wpForo uploaded images. But this will not work for the Advanced Attachments addon images.
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There is a plugin named Dynamic Watermark it applies watermark to the whole website no mater where photos are stored and you will have to exclude header and other images from watermarking because the plugin is no longer supported his creator did not implement image exclusion as you know no one wants to work for free.