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.