Completed solution (unless there are more loose ends to tie up!)
When the image is downloaded
the transparency mask spells out some words in the empty cells - when viewed on a black background it looks like this
![Initial image with letters visible](https://cdn.statically.io/img/i.sstatic.net/6851Y.png)
Specifically
(with spaces added for readability)
JACK OF ALL TRADES MASTER OF NONE OFTENTIMES BETTER THAN MASTER OF ONE
I would interpret this as
a clue that this isn't a puzzle for an expert sudoku solver, but one that requires solving several other types of puzzle at the same time - possibly involving the letters as clues, or the pictures, and almost certainly a not-too-difficult sudoku being involved too...
But as others who are more aware of what's going on round here already knew
it was (also) a reference to this very puzzle being the 200th sudoku puzzle and thereby unlocking the Generalist badge.
The partial answer given by @Jens
explains how each of the images is linked to the word 'General' (which also indicates I'd correctly identified at least "A Major" and "Failure" before the relevance was understood). The tank shown in an earlier revision could also have been linked, either by a military connection, or having been named after a general.
The textual clue in the question "The proper word to use as the ninth symbol is in Berlin (3)"
"... is in Berlin (3)" hints at the German word for "is" i.e. "ist", being the word that, combined with "General" gives "Generalist". (Also credit to @Jens)
"The proper word to use as the ninth symbol" - the word that links all the others could be considered "the proper word".
Putting both together makes "GENERALIST".
Completely solving the sudoku part (for details of my solving path, see revision 3 in edit history), I get
![solved sudoku grid](https://cdn.statically.io/img/i.sstatic.net/GVaRS.png)
This leaves the following letters uncovered: C, D, S, I, O, T, E, S, E
Those letters include the -IST, leaving C, D, O, E, S, E
... but the letters subsequently seem to have been a red herring - being used only for the poem.
Relevance of Lamborghini Rebenton:
can be explained with a clip from an old episode of Top Gear (also linked from here)
"... but with this you would get a lot of badge".
In context this is another reference to the generalist badge that has been unlocked by this puzzle being posted.
Putting it all together:
I can only assume that the proper image to use for the 9th symbol is the generalist badge itself. In part, because it explains why the grid was so big in order to make it fit!
![Final solved sudoku including Generalist badge as the 9th symbol](https://cdn.statically.io/img/i.sstatic.net/DShid.png)
A further note on the convoluted solving paths:
The puzzle attempts to have some link, however tenuous, to 20 of the top 40 tags. If they were all actually used on the puzzle, it could single-handedly qualify for the generalist badge in its own right.
Dead ends and/or speculation: - now tidied up - see revision 11 in edit history