I don't know for certain that these will work the way you intend but Ember claims to take screenshots of entire web pages. I'm not sure how it works with iframed pages.
If it is not critical to get a screenshot of the content of the iframe with that background image you can use Firefox to open the iframe in a separate tab and then get a screenshot of it or, if it still scrolls, Print it and choose a PDF program as the printer (you'll need Adobe Acrobat, CutePDFWriter, or something similar to have that option).
If it IS critical to have all the other page content visible along with the iframed content, you may need to add a CSS change to the PDF print option. It may be possible to edit the print styles of your web site so that the iframe is taller only when the page is going to be printed. That way it will look normal when just viewing it but will show all the iframed content when it's being printed (or printed to a PDF program).