And a practical example to make things easier for you guys. Let us take this pony for instance:

I would like to see how her items and her tattoo would look like on this pony:

So, I'm going to take the original code:
http://ponyisland.net/image.php?type=pet&
pet=number&size=200&
breed=EarthPony&
gender=Female&DNA=
body|hair|eyes|hair2|extra1|extra2|bodygene|leggene|hairgene|facegene|&birthday=0
First, I will go to the profile of the first above pony to search for the pet number. Her URL is: http://ponyisland.net/index.php?src=pets&sub=profile&
pet=12116232&usr=363571
So I copy that number and paste it into my code:
http://ponyisland.net/image.php?type=pet&
pet=12116232&size=200&breed=EarthPony&gender=Female&DNA=body|hair|eyes|hair2|extra1|extra2|bodygene|leggene|hairgene|facegene|&birthday=0
Now I look at the breed and gender of my 2nd pony and check the code to see wether I have the right breed and gender written in there.
Next, we check the hexes of my 2nd pony. We first hover the mouse over the body colour in order to find out the Hex:

We write that into the code:
http://ponyisland.net/image.php?type=pet&pet=12116232&size=200&breed=EarthPony&gender=Female&
DNA=4E3A16|hair|eyes|hair2|extra1|extra2|bodygene|leggene|hairgene|facegene|&birthday=0
And we do the same till we have filled in all the hexes:
http://ponyisland.net/image.php?type=pet&pet=12116232&size=200&breed=EarthPony&gender=Female&
DNA=4E3A16|90C293|41629A|785F3B|877385|9A5A42|bodygene|leggene|hairgene|facegene|&birthday=0
Next stop, s'genes!The 2nd pony, the one I see the tat and the items on, shows colorized bodyfade, legfade, freckles and rainbow hair. So, I look at the DNA list in the first post to find the DNA code for each of the genes.
Body Fade Color - DC
Fade - DD
Rainbow - RR
Freckles - FF
After I found them, I write them in the code:
http://ponyisland.net/image.php?type=pet&pet=12116232&size=200&breed=EarthPony&gender=Female&DNA=4E3A16|90C293|41629A|785F3B|877385|9A5A42|
DC|DD|RR|FF|&birthday=0
Finally, I place my entire code between the IMG tags and voila:
