Translation:I will take one hundred pictures.
You wouldn't normally say that in English because taking 100 pictures is a process that takes time. It would be either "I am taking" or "I will take" to be natural in English and the "-ing form" is a different conjugation. Thus, "I will take 100 pictures" is the only translation accepted.
The verb decides the tense for the sentence. The verb is in present tense so the sentence is in present tense, BUT the present tense in Japanese can also be translated as a future tense because it could refer to something you are either yet to do or are in the process of doing/about to do, if that makes sense? A good example would be ashita gakkou ni ikimasu - I will go to school tomorrow. OR sarasishuu no kinyoubi ni aimasu - I'll meet you on Friday, the week after next.
～まい is a suffix counter for flat objects like paper or bread or cardboard or photos (as in this instance). You may note that ～まい is attached to 百 - one hundred, so in this sentence it tells us how many pictures (flat things) the person is taking. I'm not sure what you're talking about when you say that い is for people and が is for counting animals - perhaps you mean that います is for living things/animals/humans while あります is for inanimate objects. Also が is not exclusive to counting animals.
My keyboard is working again - YAY!!
It is before the verb - it follows the direct object of the verb. The reason why it does not directly precede the verb is because between を and the verb or the last particle and the verb in a Japanese sentence is where quantity of an item/s or number of people is typically placed. In this instance 百枚 - 100 flat things (ie. photos/pictures) ねこ が ３びき います - There are 3 cats. 本棚 に 本 が ５さつ あります - There are 5 books on the bookshelf. いもうと が 2人 います - I have 2 younger sisters.
It was marked wrong because "take 100 pictures" is a command. The original sentence is not a command. The verb is present active. It could I/you/we/sh/he/they take 100 pictures but it is not "take 100 pictures". This form of the verb requires a subject - someone who performs the action - the taking of the pictures. If you think that a subject must be explicitly stated then you do not understand the Japanese language. Where a subject is not stated it is implied - usually I/the speaker or you/the listener if the sentence is a question.