The Open menu item, Cmd-O, brings up a standard Open dialog box. Multiple files can be selected. Files selected are added to the table on the left side of the window. Duplicates are not allowed. An attempt to add a duplicate fill will result in this alert:

Individual images can also be dragged to the end of the table of files to process. Non-image files either selected in the open dialog or dragged to the table will show up in gray instead of black. Gray indicates a file type that GeoTag does not know how to process.