Unless this "someone else" has physical access to your phone, it is very unlikely that he is directly deleting pictures or files from your phone.
However, if you are synchronizing things like photos and files through services like iCloud Photos and iCloud Drive (Apple) or Google Photos and Google Drive (Google), and this person has the password to your account, it would possible for them to edit or delete your data elsewhere, and have those changes show up on your phone.
If you think that anything like that is going on, you should change the passwords for all affected accounts.
If you think your Apple Account has been compromised - Apple Support