I am sorry I didn't get to set up a test on this earlier but I have done so now.
I discovered it is not Rh that is converting the images to JPGs, that is done by Word before Rh takes the file.
Take one of your Word documents and do a Save As. Choose the WebFiltered option which converts it to an HTML file which is what Rh needs to import. You will find that some of the images are still PNG files and some have been converted to JPGs. In all cases the conversion to JPG is where Word has had to reduce the image to fit the page width. Tables are not of themselves a problem, images there only get converted to JPG if the cell width forces Word to reduce the image size.
Word reducing the image size is the issue. In a copy of the source document that I used for the test above I went to the images where Word had reduced the size and clicked Size and Position > Reset. I saved that document and then imported it. This time all the images were PNGs, same images.
I don't know what you use to grab images to put them into Word but SnagIt for example allows you to resize before you paste into Word. You can do that after capturing or even set the capture to automatically do that as part of the process. By keeping alll you images 100% in Word, you would avoid this problem.
See www.grainge.org for RoboHelp and Authoring tips
Thank you for your help Peter. The timing worked out well as I'm just now getting back to converting Word docs.