Updating thumbnails in NetSuite requires the usage of an external restlet. The restlet used was developed by Tim Dietrich and is freely available at https://suiteapi.com. This is optional. Without it no thumbnail uploads are possible.
SuiteApi is a set of functions which assists an authenticated user with uploads of files using the NetSuite REST api. File uploads are not supported natively, and this set of scripts extends the functionality.
If for some reason the website below is unavailable, a copy of the Suite API zip file can be found here: SuiteAPI-v2022.1.zip Follow the steps in the SuiteApi setup
** If the above link / website cannot be found, a copy (current at the time of writing) of the file and the instructions can be found using this link