Phone background image help please :-)

Hi all I have got a package with a few sims phone background covers but i want to add more to the package. but i cant work out how?
They are DST image files.
The import export options and Batch export create a file but when i edit it and put it back it dosnt work.
If i duplicate one how do i change the instance number so it is difrent so it will add it?

Hope im not being to confusing and hope someone can help
best wishes
Steve