Conversion of Images from HTML Files

I have been trying to convert a PDF book with hardly any text into an EPUB and MOBI format. I converted all of the PDF pages as high-quality JPEGs.

I inserted the JPEGs into a Word File and saved as a Filtered HTML page, which I have done before with another book with excellent results. This time, however, a lot of the images became blurry.

I went back and saved them as PNGs and TIFFs. These looked very clear in Word but improved only slight after converting them using Calibre. (i.e. still not significant enough to release to the public)

Has anyone any suggestions as to what I can try next?

Thank you,
SG