I'm going to write up an employee
What is the reason for writing this employee up
She's been stealing from the company for a long time.
It's understandable to report wrong doings in your company. Just make sure you have the right facts and can back it up when needed.
I've already prepared for this, I even added a photo of the employee, watching him on his phone when he was supposed to be working.
If you have evidence of this employee goofing off on the job, then it is fine for you to formally reprimand them.  However, if you are going to accuse them of theft, you will need actual evidence of that crime, not just a photo of them on their phone.