How do you fade a picture into a picture on Photo Shop Elements?

Ok, i have Photo Shop Elements and i want to learn how to fade one picture into another, like you have one picture- of lets say an eye- and then you have a picture of- lets say water- and i want the to be able to fade the eye into the water so when you look at it it looks like the eye is in the water- or something along those lines. i just want to know how to fade a picture into a picture on PSE. i don't blame you if you have no idea what i'm talking about but if you know how to do this please tell me! thanks!