How can I clear my phone's hard drive quickly?

Galaxy S3. It was difficult to charge and died quickly. Got a new battery, same problem. Got a new phone. This was two years ago.

I want to get a new phone tomorrow, and turn in my old phone (rather than my current) for $100.

I had some nude pictures of my ex on my phone. I deleted them, then used the phone for about 7 months before getting a new one.

I don't know if those pictures are gone forever or not.

I don't know how long my phone's battery will last (I managed to get it to charge overnight thankfully). What is the best way for me to make sure the pictures are gone forever before turning the phone in? Are they probably gone after 7 months of use?
Added (1). My ex and I are still friends, and she's pregnant. Now that she's going to be a mother I definitely don't want those pictures getting out.

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