I tried to keep the game is easy as possible with the Python Turtle module and added simple gif animation for the aliens. I am making a collection of easy games in python for teaching my kids. I also added a game class allowing start and game over screen/replay. Feel free to look at the code below for any improvements/questions:
https://www.dropbox.com/sh/d6yksmi6yu...
See the youtube tutorial by Christian Thompson!
For the images I did a google search and did a few screen shots. Images were converted to gif files and resized online at ezgif.com.