So what are they? They are stickers that you put on your baby's onesie each month. Then you take their picture, and you have pictures of them for every monthly milestone during their first year. My favorite are these (for boys) and these (for girls) from Picky Sticky.
I use a long sleeve white onesie and put my baby in the same chair for each picture, that way I can see how much he has grown! Here are the ones I have taken so far:
{all photos by me}