How do I add a transparent bubble to an image with Photoshop?

Kind of impossible to answer without knowing exactly what the picture of the bubble looks like (is it on white, on a solid color, over another image, cut out, with some transparency already, etc.).

But your best bet to start with is to put the bubble on a layer above the picture and then play with the blending modes (the pulldown at the top of the layer palette - the default says Normal) - one of them may do exactly what you want without having to do anything else. If there is a background, you might want to get rid of it so you are working with just the bubble.