particle に - 今までに vs. 今まで - what is the difference in meaning?

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?

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.