Could you tell me if this sentence is right?
The pilot saw one of the men WAS taking photos.----------Should I put WAS there?

Question

Answer

Sorry - posted a comment instead of an answer.

I agree with the first comment; using "was" there is correct. 
These may also work:

The pilot saw one of the men had been taking photos.

or...

The pilot noticed one of the men was taking photos.

Answer

Yes, using "was" is correct. The singular form is necessary.

Answer

I agree with the first comment; using "was" there is correct.

These may also work:
The pilot saw one of the men had been taking photos.
or...
The pilot noticed one of the men was taking photos.