Thanks for your clarifications. Not sure why is Plex isn't displaying the actor image when it is stored locally. But if can scrape the nfo to include the thumb url instead it can display on both KODI and Plex so downloading the image will become optional. For example like this below.
<actor> <name>Nishimiya Yume</name> <thumb>https://pics.r18.com/mono/actjpgs/nisimiya_yume.jpg</thumb> <role>Actress</role> </actor>
The interface for Plex and KODI is very different. Plex is more suitable for PC using with keyboard and mice whereas KODI is more for TV with remote. They are both very dominant and have their own userbase. Plex is a great enhancement for KODI for those who prefer PC. It has more metadata to display, better sorting and filtering etc. You should explore Plex and give it a try, if possible support for both. Thanks!