I think you can set that in JMP Preferences in the Windows Specific section. The copy/drag graphic formats will allow you to always have 300 dpi pictures when you copy or drag JMP stuff.
There is an option to save files with high resolution (300 dpi) on Windows machines. Go to File > Preferences and choose the Windows Specific preference group. In the Copy/Drag Graphics Formats section, select 300 from the drop down menu next to "Resolution (DPI) for PNG and JPEG images."
You may also try the Set Output DPI available on the JMP community file exchange: Set Output DPI.
I have an easy Mac solution, for those (like me) who are still searching this question and come upon this old thread.
1. Copy the graph in JMP (either the one you've created in Graph Builder or from any analytical output)
2. Open Preveiw (Apple's built in PDF & image viewer).
3. Open a new Preview window (File > New from Clipboard, or use the shorcut ⌘N).
4. Next, choose File > Export
5. Choose your preferred format (options are HEIC, JPEG, JPEG-2000, OpenEXR, PDF, PNG, TIFF as of Sept 2019)
6. You can adjust the quality and/or resolution for all but PDF formats.
Even though this is an old thread, I still had this question this morning (Sept 2019), so I hope this helps other Mac users out there!