Wouldn't really go any lower than 300 DPI scans (once you start getting lower, the accuracy of OCR begins to drop drastically).
As to HTML tables versus image tables, there was a large discussion here:
https://www.mobileread.com/forums/sho...d.php?t=223062
I posted examples + reasoning for the HTML Tables side. I believe the benefits of scalability + searchability, highly outweigh the "looks" + bloated filesize of an image.
But if you MUST use an image, please avoid JPG like the plague, and save it either as a GIF or PNG (preferably PNG).