The original poster asked about a replacement cover with a photo on it and specifically stated they did not want a case. All the answers above are for cases - not what she wanted.
I think a closer option is to use skins. These are vinyl labels that stick to the existing cover. I've use skins before on some of my phones to change their appearance and they work pretty well.
There are several vendors that will do skins using your photos. The below is a random link to one I saw using google search. I have no experience with this vendor.
Design Your Own Samsung Galaxy S4 Custom Skin - Samsung Galaxy S4 (S IV) Custom Cases & Skins