If you mean in this forum.. then no.. you will always have a link be a hyperlink'd word or picture.. because the forum filters out any html in posts.
In theory you should be able to put a link on a photo like I said before.. but the photo you can not see because its is transparent or the same as the background. Post a photo using img tags and put the link in there. Look at some of Frogberto's photo threads for examples.
But again.. what are you specifically trying to do? Hidden links in someone else's website would probably be considered malicious if not done for a good reason.
well that is something you should ask at neopets, as we would have no idea what their website is running.
The theory remains the same... a link always needs something you can click. That is going to be a word or graphic. What people do is make it so the graphic can not be seen... that is how you make a link be not visible unless you click it.