A lot of pictures I work with come from www.foxes.com -- this helps because they usually have a "regular" and an "extralarge" version of the same picture, so I can copy the "extralarge" breasts onto the "regular" girl.

Regular	Ex-Large
Click on pictures for larger pictures

If you don't have such a convenient pair of pics, you can enlarge copies of the original breasts as well, as long as you clean up the edges (I often then put copies of the original nipples over the newly enlarged breasts as well since any blurring is most noticeable on them).