Does anyone know of a editing program to add in subliminal messages?

For a psych project I want to test the threshold of peoples abilities to see subliminal messages and if seeing one but not realizing it will trigger different emotions or feelings. I'm looking for a program to simply edit in either a message or one to create a flash animation that I can place a small word or image into that will appear and disappear very quickly. As I am terrible when it comes to computers a simple one would be preferred.