Since "pictures" are countable objects, it's better to say "a significant number." This is treated as a plural subject, and the verb should be "were":
A significant number of these pictures were taken at the museum.
Use "amount" for something that has quantity, but is not counted as individual objects. If "amount" is the subject, then "was" is the correct verb, as you say. For example:
A significant amount of money was missing from my bank account.
I hope this helps!