At the Pennywise most our graphics department uses inDesign for most image production.  In the past, we would open the final pdf, then convert them to a jpeg for the web using Preview or Adobe Acrobat.  I was always finding the images to come out with terrible artifacts and choppy lines.

The best practice is to open the original inDesign file, and simply export it as a jpeg with maximum quality, progressive baseline, 300ppi, and (because this is for the web,) RGB colour space.