When Exporting PDFs in Bulk, images in the print format are not exported to the PDF.
(please note this is not a letterhead, this part is written as HTML in the print format itself) (Images from Letterhead are exported to the final file correctly!)
Also, another issue is when exporting bulk PDF for any DocType, the exported result is a single PDF with all the individual entries clubbed together, which in a sense defeats the purpose of exporting in bulk, because, in the end, you will have to separate these individual entries with an external PDF editing tool. (it would be better to have an option to either bulk export all the entries in a single file or export them to separate individual files.