You can use any of the pictures from Neopets as a basis for your pet pictures, or you can create it all from scratch. Just make sure that you are using your own original work and not copying from someone else.
If you want to use an image from the site just save it onto your computer and open it out in any paint program. Once it is open draw on the clothes, make up, hair, etc. that you want your pet to wear and then save it.
If you are using a PC you can right click on any picture you see on the site to get choices for saving to your computer.
If you are using a Mac, click and hold the mouse down over any picture for choices.
You can find pictures of your pets on the Quick Reference page in Pet Central.