You may have already tried changing the texture via the Page Properties dialog box, but let's try another way. Click on the Textures Tab at the bottom left of the screen (this is the red and black hashed icon). Now drag one of the textures in this list onto the page. You can also double click it, of course. And there you go! You have a textured page.

Note that this texture will be saved in the .phka photo album file (unlike photos themselves), and so anyone you send this album to will be able to view the page properly, even if they don't have this texture.