Hi Jerome,
I think this is due to the different image properties. The dpi basically is used to calculate the print size of a given image.
The new 768x733 pixel image with 72dpi has a print size of about 271x259 mm (10.7x10.2in) whereas the old 3300x2463 pixel image with 600dpi has a print size of about 140x104 mm (5.5x4.1in). Therefore your new image runs off the page.
So I guess your best bet is to use some image conversion tool to correct the print size of the images to an appropriate size by modifying the dpi settings.
Hope that helps,
Sirko