NX only supports 'thumbnail' previews, which are adequate for use in the NX File Open dialog and for the .prt file icon displayed inside the Explorer window itself. Unfortunately, to limit the impact, of including this image, on the NX part file size, the overall size and resolution of this image saved inside the NX part file was not suitable for use in the Explorer's 'preview pane'.
BTW, you can also capture the thumbnail 'snapshot' image manually if you wish and then have that image used instead of having it updated every time the part file is saved. This is handy for things like standard parts where you might like the thumbnail image to show the part in a certain way with a certain orientation and background for clarity but which would not necessarily be how you'd like to see it on the screen when it was opened. A good example is something like a threaded feature which would LOOK nice if you saw the preview threaded so as to help identify the correct item, but where you wouldn't want the part to come in with the detailed threads displayed for performance reasons. You could enable the threads, set the background color and orientation and scale as you would like it to appears in the preview image and then go to...
File -> Properties -> Preview
...and you will see the current preview displayed (if the file has been saved at least once). To get a manual preview, just change the 'Creation Time' option in the 'Part Preview' section of the dialog from 'On Save' to 'On Demand'. Now get yoru model to look the way you want it, then select the 'Create Preview Now' button and you'll see the new preview. You can repeat this as many times as necessary until you've got the exact image that you want. One you're satisfied, you can close the dialog, set your model back to how you want it to be when used, such as disabling the threads, changing the background color, whatever, and then save the part file. From now on, no matter how many times this file is opened and modified, when you save it the preview will NOT be updated and will remain the way it was when you manually captured the image.
Here's an example of what I mean:
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:
To an Engineer, the glass is twice as big as it needs to be.