Hidden images, cellphone stolen is someone able to get the images?

Hi!!!

Unfortunately my cellphon was stolen. I have several pictures on it in the internal memory, I think someone will install a new fresh android room. But... my question is, is someone able to access to my galery images?
Also I have some content hidden, and I'm worried about. If someone access to the phone memory is able to see this hidden by code pictures?

Thank you very much!!!

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