to visually sort images under Linux doesn’t seem to be a trivial task. I duckduckgo‘ed for a long time and had a look at various image and file managing applications before finding gthumb. And even there, you first need to create a “catalog” and within the catalog a “library” which will finally allow you to manually sort your images. All of which is not documented.
Once you’ve sorted your images, you’d possibly want to export the sorting? Again, no trace of any help or documentation: gthumb catalogs are saved under $HOME/.local/share/gthumb/catalogs/foobar.catalog.