Sorry... I don't understand what you mean. This plugin has nothing to do with face icons, it adds functions so that you can create summon skills for enemy units.
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.