First you need to set up the rank images using the ones on the starfile image page.
Admin > Membergroups
Modify the group you want to do this to.
You will see the screen below.
Click on the starfile image link.   (underlined in red)




Choose one of the images  (it doesnt matter which ones you choose as you will be swapping them)

Enter the filename for the image you chose in the star image filename blank for the membergroup.

You need to know the url of the image so you can swap it.
Right mouse click on the image - Do this on the starfile image page so you have the correct url.    
Click Propertys.
You will see the url there.
Copy the url.


To make the code to swap the image go here:

Swap Images Generator

Put the url of the starfile image under OLD IMAGE
Put the url of YOUR image under NEW IMAGE

If you want to do it for more images click ADD NEW FIELD and put in the urls for the additional images, old and new.
   
When you are done click the Get Code button.
Copy the code.
Paste it into your Footer   (Admin > Manage Styles > Edit HEader/Footer)