Saving large webpage as Image
Print to PDF, then save the PDF as an image. I think that's probably your most foolproof method.
Printing to PDF is straightforward and you've got a ton of options, conversion after the fact will maintain page layout.
Conversion answer here: How do I save an image PDF file as an image?
As a note: if you use the PDFCreator project available from http://www.pdfforge.org/ the option to print a PDF as an image, or to print as an image is native.
The Firefox Abduction! add-on couldn't save an image that large. If you absolutely want to save an ultra-lengthy single image and are open to non-Chrome solutions, here are a few that might work:
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SiteShoter is a portable app:
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There are any number of online web screenshot apps, such as this one