I think it would actually be easier and more preferable for them to start on the T-shirts. That would cover a lot of clothing items and could be done relatively quickly. They could, for example, start with one pet - say, an Acara - and put/draw/trace, say, a Red Marbleman T-shirt on it. Save. It would be a very simple matter to change the colors of the sleeves and character on the front to Yellow, Green, and Blue in Illustrator; I assume it would be in Flash too. Save each one. Bingo, they've got a common wearable item in four different colors for one of the most popular species of pet.
Then, repeat this process for the rest of the pets. Though it would still take time since there's a whole lotta species. They could ease complaints by doing the most popular species first. =p
In fact, they could do almost all the T-shirts (and hopefully Altador Cup jerseys, eventually
) in existence from one basic template, just making changes in color and details. Now, some admittedly wouldn't be quite that easy - some have shorter sleeves, others have ragged edges on the bottom and/or the sleeves, some have really elaborate designs - so those would take a little more time.
The customization program is cool, and I think a lot of great ideas, designs, and Neopets characters are going to come from it. But I really wish TNT had waited a little while longer before releasing it, because right now there's just not a whole lot to DO with it unless you can afford multiple paintbrushes or have access to the lab ray. So far I've only been able to put backgrounds behind my pets, take off my Royal Acara's princess hat, and put a Blechy hat on my Xweetok.
.....yay?