Since an Image Backup shall include the Oracle home directory, the OraInventory directory, the oratab file, and Windows registries on that node plus a cold backup of the target instnace... I'll say you have to shutdown your instance and start using the copy command ;-)
How long to copy 100 GB?... it depends...
...are you making the copy on a disk that it's visible from your base server?
...are you moving the 100 Gigs over a network?... how fast is your network?
I'm confident you can go ahead and answer your own question now.
Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.