If you mean make them grow on their own, I'm afraid you'll have to deal with some serious coding, which Nick has made a tutorial for in the Help section. I'd give you a link, but I can't copy on my Nook. ^^;
You can simulate growth by using Photobucket (which uses the file name in the URL, which is necessary for it to work). You name each stage the same name and put them in separate folders on your computer. When you want the pet to grow, simply delete the pet you want to change off of Photobucket and upload the second stage. C: There's a tutorial somewhere in Share Your Creativity, I think, but again... I can't copy/paste. :c I can give you the links once I'm back at my laptop, if someone else doesn't post them first. ^^;
Edit: let's see if this works:
viewtopic.php?f=10&t=77860