To change a Cover Image, go to a Gallery, hover over the Page cover you'd like to change and click on the pencil icon that pops up over the cover image. In the menu that appears, click "Edit Cover Image".
After clicking Edit Cover Image, the Cover Image Cropper will appear in a dialogue box. If you already have a Cover Image, you'll need to click "Remove" on the top right. Then, chose from your files the new image that you want to use. When you're all done, click Done or click Cancel to undo the changes you've made.
After you've selected the Cover Image you'd like to use, you can further customize how it appears on your Portfolio by clicking on the pencil icon over the Cover Image and then selecting "Customize". Alternatively, you can select Page Covers option in the Remote.
The dialogue that appears allows you to make a number of customizations including rounded edges, colored overlays, and rollover effects.