No se puede, si leyeras la FAQ de coppermine te encontrarias con:
Cita: How do I remove/edit the bottom line "Powered by Coppermine" ?
You mustn't remove the line - it's part of the deal: you're allowed to use Coppermine on your site, but the line stays! It's more than justified to give credit where credit is due; be glad you're able to use such a great piece of software for free.
You are allowed to change the way the line looks by editing the CSS class to make it fit into the design of your site; edit /themes/yourtheme/style.css, look for the class "Footer" and change it as you wish.
Note: don't ask stupid questions on removing this line in the Coppermine Photo Gallery Support Board - your posting will be deleted!
pero como puedes cambiar el CSS puedes hacer que el texto tenga el mismo color que el fondo y no se habra desaparecido pero seria invisible