You almost certainly own the copyright already, if you took the picture. You are the author of the work, you don't have to do anything.
The only common exception is that if you are in employment (eg, employer pays income tax on your behalf is one easy test, though there may be others - as opposed to freelance) and the photos are in some way connected with your work, in which case your employer owns copyright.
Employer's rights are far more extensive than many people know, and it doesn't matter what you are technically employed to do, or where or when the images are made, eg out of office hours, off work premises etc. In answer to the question, 'would I have been able to take these pictures if I didn't have this job?' if you answer 'no' then you probably don't own copyright.
However, if you answer yes to that one, and have not formally assigned the copyright to anyone else, then they're automatically yours.