Before finalizing export, please choose an Identifier for your pack. This will allow the Portrait Rework mod to identify your mod so your Portraits aren't accidentally overwritten. This Identifier will then be appended to the end of the file names.
If you aren't sure what to use, a good suggestion is to use the ID of your mod in its package.xml
If you ARE trying to overwrite Portraits from another mod, simply input the exact same Identifier they used for their images here.