Yeah, I can't do any code which involves converting text to an id and so on - this means I can't do a [neopet species= colour=][/neopet] tag, since it's no longer just a case of subbing a colour and species into a URL. It has to be something where the text you enter can be used exactly as is.
It occurred to me that it would be possible to do a petimage tag- using the http://pets.neopets.com/cpn/PETNAME/1/2.png url format (note the "cpn" as opposed to "cp" in the usual urls). It always resolves to the version with the jumble of 8 letters and numbers which is common to all pets of that species and colour wearing the same clothes.
I don't know if there would be any demand for it, however, and we don't want an excess of custom tags which clutter the form up and don't get used.
I think, though, that those are the images which don't update properly unless you go to Customisation - since a lot of pets for adoption are lab pets, I'm not sure that the images which get displayed will actually be accurate.