Is it okay to directly copy a diagram/image from another research paper into yours?
If the image/research paper is marked as released to the Public Domain, then you can use the content as you wish. That being said, if the image is actually from someone/somewhere else, you may still be able to use it as per the Fair Use Act:


These sites may help you understand more:


Stanford University - Fair Use Overview
FairUse @ US Department of Copyright