In order to gain access to said photo's one would need at least remote access to another persons computer. Computers can get infected with malware, trojans or virusses which enable back door access, mostly these back doors are used to send large amounts of unsolicited e-mails or to send out requests to overload a server (DDOS-ing). Using this back door access you could see what is going on in terms of datatraffic and application usage, you can even log things typed on the keyboard or get screen-captures of things going on on someones display.
To prevent things like that happening, you will need to update your operating system, use a good and up-to-date anti-virus solution as well as a firewalled internet connection. Smartphones are usually forgotten when updates arrive, and have become a target as well.
ps. With "dark web" you mean TOR?