Units silhouette images from https://openclipart.org/ (public domain):

https://openclipart.org/detail/17966/soldiersilhouette
by drunken_duck: https://openclipart.org/user-detail/drunken_duck

https://openclipart.org/detail/221992/soldier-with-rifle-silhouette
by GDJ: https://openclipart.org/user-detail/GDJ

https://openclipart.org/detail/211399/alien-head
by Sev: https://openclipart.org/user-detail/Sev

https://openclipart.org/detail/174910/alien-egg
by artbejo: https://openclipart.org/user-detail/artbejo