Not using %d in the singular form of English does not prevent other
languages to use %d in any form they wish to. This will still work and
will still be replaced by the relevant number of images.
So I revert commit a0724783 because it is just prettier (in English)
to write "An image" rather than "1 image", but this does not mean you
have to do the same in other languages! Adding a comment so that
translators know about it.

Also directly modify the msgid in the Polish and Russian translations
which already translated this string, so that the translations does not
end up unnecessarily fuzzy.