He's talking about the thumbnail of your plugin, you can see the actor's face portrait in the hud at the bottom of the screen. I'd like to know the plugin aswell if you have it.
Are you sure you're talking about the Enemy Reinforcements plugin?
Because the thumbnail here has no actor's face portrait visible. oO
However, whichever thumbnail you are referencing, it must a default feature of either RPG Maker MV or MZ, as all images I capture from my plugins are taken from my test projects, with only my plugins enabled. I do have Yanfly's/Visustella's basic plugins loaded for testing purposes, but they are usually disabled when I finish testing and take prints.