Ya real simple. Look up what the ‘google pixel’ or ‘Facebook pixel’ is.
Basically they are 1x1 pixel images that are used for advertising purposes. However, a regular photo can be made to function the same. When a user download or opens the pixel, it send a packet out to a server and tells the server (via ping) that it was opened or downloaded.
A hacker could probably make this work as a Dos. Maybe even script it to where it continues to ping. Idk if they can send out more than one ping or not. But it will ping each time it’s opened.