[ticket/12666] Use "None" for images in root of gallery avatar path

"None" serves no real purpose right now. After this it will be used instead
of "Main" for images in the root of the gallery avatar path.

PHPBB3-12666
Merge pull request #2552 from marc1706/ticket/12666

[ticket/12666] Use "None" for images in root of gallery avatar path

* marc1706/ticket/12666:
  [ticket/12666] Use "None" for images in root of gallery avatar path