We've added batch resizing to the "Manage Album Images" page - just go to the album that has the pictures to be resized, and look for the blue "Manage Album Images" button on the top right, under your cover photo.
Click that, enter the maximum dimensions (width and height) for resizing, hit the checkbox for each image you want resized, and then click the blue Resize button above the images.
This will not over-write your existing images, but instead create new copies, at the maximum size you...