Your TSV file should look something like this (where <TAB> is the thing you get by pressing your tab key - most likely sitting to the left of the top row of letters).
Filename<TAB>Short name<TAB>Gallery<TAB>Description<TAB>Licence banana.jpg<TAB>banana<TAB>fruit<TAB>Our favourite yellow fruit<TAB>cc-pdIf you had any pending uploads, you could download a skeletal TSV file to get you started.
You can just do this in a text editor - but you quite likely have something which will make the job easier. Spreadsheet programs like Microsoft Excel or GNU's Gnumeric will let you save the grid as TSV directly - giving you a more reasonable interface to edit in. Remember to save as TSV though - this won't work if you send in an Excel document directly.
Note that no fields can contain tabs themselves. Since a tab will never be much use in any of the fields - this isn't too harsh an imposition.
The top line of the file denotes which columns you are filling in. You can fill in any of the following. You must include either the filename or the numeric ID (see below), as this is the most sensible way to identify the picture until you give us a short name and gallery where the picture will live. You should check that you are spelling these correctly, or the next page will complain (capitalisation and spacing does not matter, but the letters do).[Note]
cc-pd for Public Domain, cc-by for Creative Commons, with Attribution.To make the pictures public you will need to fill in at least the Short name, Gallery and Readable name - if you haven't already.