I saw some palettes created by CL users from a photo in the gallery. How can I try my hand in Photocopa on the same photo? Linking doesn't work, as the link doesn't end with a file, but with a slash... I can't think of a way to do it, and searching the forums didn't help.

You have to browse through the Photocopa gallery to find the photo you want. Right now, that's the only way.

Adding a search feature to the gallery and adding a make a palette from this photo button to the photo page are wishlist items that will hopefully be added to the next version of the site.