Choose a photo whose shape and orientation work well with your jar. To use a bottle, you'll need to pick a photo that is only as tall as the bottle's widest point. Crop the photo as necessary and roll it slightly so that it will fit in the jar. Insert it upside down and adjust it with your fingers or with a pencil eraser until it is properly positioned. Then turn the jar over and voila!
The thickness and curve of the glass creates an interesting fish-eye kind of effect. Arrange several jars and bottles in different shapes and sizes for a pretty display.
For more detailed instructions with step-by-step pictures, visit Photojojo.