How to control the thumbnail generation when saving assembly data in NX.
Solution
If you do not want to generate thumbnails by default for all parts, then you can open the Template part and go to its File > Properties and Remove Store Preview like below snap and save the template Part. See below snap.
If you are using Blank template while creating the part file then you can control the same using Customer Defaults, File tab >Utilities > Customer Defaults > Gateway (in the left box)> General > Part tab > Part Previews options
Notes and References
9493956
Hardware/Software Configuration
Platform: INTL64
OS: windows
OS Version: 764sp1
Product: NX
Application: NXMANAGER
Version: V12.0.2
Function: PREFERENCES
Ref: 002-8016459