The city-sized JMC spaceship Red Dwarf had many labs, some of which were photography processing labs. This dimly lit lab could process various formats of pictures including slides, and consisted of projection screens and work benches with various chemicals and developing fluids. It was not uncommon for people to end up with other people's photos, as lister found out when ended up with someone else's Skiing pictures.
When the lowest ranking crewmember of Red Dwarf, Dave Lister, smuggled in a pet cat onboard, Holly detected the presence of the cat, but Lister kept it safely hidden. However, Lister was discovered when he took photographs of himself with the cat, and sent the photos to the processing lab to be developed. When Lister refused to give up the cat, Captain Hollister sentenced Lister to 18 months in stasis. (Pilot episode)
Three million years later, when Kryten took some photographs of himself having a party with himself, the images in the photographs come to life before his eyes as he developed them. Holly surmised that the developing fluid Kryten had used had mutated over the last three million years. When Kryten converted some of the photos to slides, the projected slides also came to life.
When Lister walked toward the first slide, which was of Frank Rimmer's wedding to Janine, he found that he could enter the photograph and interact with the wedding party. Frank found Lister's intrusion annoying and began to hit him. Lister tried to walk away, but found that he could not leave the frame of the picture. He had to escape by the way he had entered, through the lens of the camera.
Other slides created included a random couple's skiing holiday and one of Hitler's Nuremberg speeches. When Lister stole Hitler's briefcase, he inadvertently prevented an assassination attempt on the Nazi leader. The Boys from the Dwarf discovered that they could use the Timeslides to alter history.
Lister used a picture of his teenage self to make himself a millionaire. He gave his younger self the idea for the "Tension sheet", a simple but lucrative invention developed originally by Rimmer's schoolmate Fred "Thickie" Holden. Lister then vanished to live a new life as a rich industrialist. Kryten and The Cat also disappeared, since Lister was responsible for their being on Red Dwarf. (and in the case of The Cat, the existence of his entire race)
Rimmer decided to beat Lister at his own game. He used a picture of his boarding school dormitory to create a timeslide and went back to give himself the Tension sheet. Unfortunately, Holden overheard him talking to his younger self and reached the patent office first. The timeline went back to much the same way it was before, except that somehow Rimmer was no longer a hologram. This did not last very long until Rimmer blew himself up. (Timeslides, Series III)