Visit the project page and click the blue “Manage" button that appears next to your pledge amount. Enter a new amount in the pledge amount box.
(Example: Let's say that you pledged for ElDorado (signed) which is 75€. If you want to add a giclée would be: 75€ + 12€ = 87€. You have to pledged the total amount to 87€.)
Later on, when the project finishes, we'll ask you which is (or are) the giclée you wish to have. :)