If you insert a web image to a page whose url contains the text download/attachments (such as this image http://confluence.atlassian.com/download/attachments/139467/confluence-3-0-profile-view.png?version=2&modificationDate=1243658607572), and then export that page as pdf, the image will not resolve, and you will get a warning in the logs

[confluence.extra.flyingpdf.ConfluenceExportUserAgent] getImageResource Could not retrieve image resource http://confluence.atlassian.com/download/attachments/139467/confluence-3-0-profile-view.png?version=2&modificationDate=1243658607572 during Confluence export : The byte array is not a recognized imageformat.

The pdf exports continue, but where the image should've been, is instead an empty space.

Workaround:

Add the problematic image as an attachment first, and then replace the web image with the attachment on the page before exporting.