thumbnail upload to Amazon Fire TV

Hello

I am using Kodi on an Amazon Fire TV with Eminence 2.0 and was wondering how I could upload images to set in my menu thumbnails. When I try to set my own image I get a fusion folder but don’t know how to manage the content of it from another computer or phone.

Sorry I am a newbie, could anyone help me there ?

Thank you in advance for any useful answer.

CHEERS