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I recently used a following sentence in my homework:

> 今日までに 私は 二百まいしゃしんを とりました。

Now, in my homework I actually used 今日まで, without に, and I got it corrected.
What's the difference between 今日まで and 今日までに?
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2011-06-14T16:45:46.313
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2016-11-11T23:09:14.620
2011-06-15T00:50:59.673
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question
particle-に
今までに vs. 今まで - what is the difference in meaning?
I believe that when you use までに, you're referring to a deadline - thus 今日までに
would be "before the end of today (you need to have finished something, you
had done something, etc)". It's referring to that specific day, in which you
took the pictures.

まで, however, is usually just used to specify that something was the case until
that time. 今日まで is referring to the time _before_ today - as in, up until this
very day, something was the case.

tl;dr  
今日まで = referring to stuff that happened/was the case _before_ today, such as
yesterday or last week  
今日までに = referring to stuff that happened before the _end of_ today

Hope that clears it up, it's a bit weird to explain.
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2011-06-14T16:50:49.180
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2015-11-18T09:32:05.120
2015-11-18T09:32:05.120
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answer
This is one of those odd examples where we could drop the に (and perhaps
replace it with a comma) and still have an understandable sentence:

> [今日]{きょう}まで、[私]{わたし}は[写真]{しゃしん}を[200枚]{にひゃくまい}[撮]{と}りました。

But this sentence, while perfectly understandable, is ambiguous on a minor
technicality, since まで by itself could be used to show when a state or an
action ceases. (Humans are smart and can figure out which one you mean, but
let's pretend we're robots. _Grammatically correct_ robots.)

> [募金活動]{ぼきんかつどう}は[来週]{らいしゅう}まで[行]{おこな}われる。  
>  The fundraising efforts will go on until next week.

The 来週まで in the above sentence shows that the action will not continue after
next week.

Since the point of 今日まで in your example sentence isn't to show that the action
of taking the pictures ceases after today, but to show _when you took the
pictures_ , the particle に is tacked on to まで just like you would add it to
any other time expression that can take に. 今日まで, though it refers to a range
of time, is here treated like a single point in time that answers the question
of when you took the pictures.

In short, the に can be dropped here. (In writing, when に is dropped, such as
in the ために and ～ずに patterns, it's often replaced with a comma.) But to erase
any ambiguity, it's best to leave it in, and I think that's what your teacher
is getting at.
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2011-06-14T17:59:02.540
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2016-11-11T23:09:14.620
2016-11-11T23:09:14.620
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answer