Vivien, go Behind the Scenes, Bulk Edit prices, then click show all images, then check image, then edit checked images simultaneously, then select all # images on all #pages. From there you will see sections that you can add prices, one of them is shower curtains. Put your markup there. Don't forget to click Submit! And wait 24 hrs for it to take effect...or it may happen right away!
Edit: Just checked your first image and it's already priced!!