Popular image formats usually have very good compression. Compare this to scanned documents, where PDF has proliferated, despite DJVU having objectively better compression and faster rendering by a wide margin.
Look at this random book from archive.org (https://archive.org/details/inferno00dant_2/
). The DJVU looks much clearer than the native PDF with more than double the size and the PDF containing JPEGs has marginally more detailed illustrations at four times the size. The text of the DJVU looks crisper than that of both PDFs.
Most books on archive.org don't have DJVUs anymore, so I usually losslessly convert a directory of JPEG2K files to a PDF and transcode it to DJVU with this script:
t=`mktemp`; d=`mktemp -d`
for i in $1/*.[Jj][Pp]2; do
jpeg2ktopam $1|pamtotiff >$t &&
tiff2pdf -o "$d/`basename $i|cut -d. -f 1`.pdf" $t
pdfunite $d/* $t &&
djvu2pdf $t > $2
rm $t; rm -r $d
This requires netpbm and pdf2djvu.