Is prime missing in photos?

[Elronnd]

Well the purpose of the exercise kind of *is* to write a prime 
number generator.  You can look up prime number sieves and 
algorithms.  For REALLY large numbers, that takes an insane 
amount of time, and you can instead use algorithms such as the 
ones outlined at 
https://csrc.nist.gov/csrc/media/publications/fips/186/3/archive/2009-06-25/documents/fips_186-3.pdf (appendix C.3)