Doesn't the "One M8" have an external Micro SD? Have you taken the card out of the phone and directly to the computer? If the pictures were stored locally on the phone, try connecting the phone directly to a Windows computer. You might need to enable USB debugging. Some phones vary on this point. Use your computer to look through all of the folders.
Dropbox is very handy for many things. Picture backup is one of them. If you create a shared folder on your home computer, all data backed upto Dropbox is immediately available on the home computer.