Written in a comment:
RetroFE is a frontend for launching old arcade games etc and the XML is used to show pictures etc on screen. Nothing in the XML actually contains any output from RetroFE. Tags in the xml are passed to RetroFE's xml engine to display images etc on screen and control animations of said images.
Based on this information, the XML file is input to the RetroFE application in the same way that a source file is input to the compiler or a DocX file is input to MS Word.
Such input files are considered to be independent works of the program that reads them. That means that the license of RetroFE does not affect in any way how you can license the input files.
Someone took one of my images and is claiming that because the software it was intended to be used with is under a GPL license, anything that comes with it can be taken and cannot be copyrighted.
The claim from that person is wrong on so many sides that I don't know where to start.
First of all, the GPL license is a copyright license, which means that it is only effective for material that can be copyrighted.
And while the GPL has a "viral" aspect, that works via the concept of derivative works and input files to an application are not derivative works of that application.
Can I claim copyright on that image and/or any other part of my addon as it does not modify any part of the original code under the GPL licensed software.
You can claim copyright on anything that you create (unless you have a signed contract that causes other rules to apply to a particular work). The context in which you create something can only dictate under what terms you can make it available to others.
In this case, there is not even a restriction on what licensing terms you can use.
Can I stop people from taking what I create just because it's used with software that is GPL licensed?
Yes, you most definitely can.