Can you clarify the question?

Example:
The user has selected an image for example "flower.jpg" (but I don't know the name the only information I have is that it was written to the variable "img");

I think I understand.... you load one or more images and the user can select one? You then want to save the selected image with it's original filename + "new".

I guess you need to store the name some where when you load the image(s); for example create a class that includes a PImage and a String - when you load the image store the filename in the string. I assume you can see which image is selected based on it's position or some other test?

This isn't really the best way to proceed but unless you can define the problem a little more closely it's difficult to offer advice (how many images, do filenames vary, how does the user select an image etc).