How do I flatten images while maintaining quality?

I am publishing PDFs for people to download and print. I am required by some clipart artists to flatten/merge their images into the document so that they cannot be extracted. I use both vector and raster graphics in my PDFs. When I tried one method of flattening using Print Production in Acrobat, it reduced the quality of my PDFs. Is there a way for me to flatten images that maintains the high quality of my graphics in either Acrobat or InDesign? By the way, I do optimize my PDFs.

Susan