Memories down, stupid question follows...

Okay, I just got Adobe Photoshop and I have NO idea how to make an animation with two different pictures. Everytime I got to try and create an animation, i can only use the same image. Is there a good website, cause I've been to a million, since I don't expect someone to take time to give me a step-by-step guide on how to do this. I want to make a transition between different images, and I can't figure out how.