Here's a pretty good tutorial with pictures. Keep in mind that the acetate shape she is using is a circle, and pretty much just fits inside the glass ball.
If you are using a bottle, you would probably need to change the shape, maybe to an oval. And be sure you leave a "tag" at the top to hang on to when inserting it, so it won't fall down too far into the bottle.
When I did my ornaments, I actually tied a length of that clear nylon thread to the top of my acetate shape and let it extend out of the glass ball a little, so I was sure the image would stay straight. What I'm saying will probably make more sense once you look at the tutorial.
Encouraging Someone: Glass Christmas Ornament Tutorial