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.