When I first got into tabletop RPGs in 1984 there were very few movies or TV shows around; I think I'd seen the animated Lord of the Rings, Dragonslayer and Hawk the Slayer at this point and other than the books themselves that really was the only visual representation I had as to how these games looked and felt.
The other imagery I used to draw on was from the huge Hollywood epics from the 1950s and 1960s. I'd watch El Cid, The Fall of the Roman Empire, Land of the Pharaohs, The Vikings, Prince Valiant and all kinds, along with movies like Jason and the Argonauts and some huge Italian classics. Along with the TV show Robin of Sherwood to keep me 'grounded', I absorbed these movies and allowed them to colour my games.
So, my first Fighting Fantasy games had the wonderful artwork of the gamebooks to give them flavour, but my D&D games needed a different injection of visuals as the few pieces of artwork in the red box Basic D&D set I had to source inspiration from another source, and this ended up being the Hollywood epics.
So, when I was describing a fallen tomb I took my cues from the great sets from The Fall of the Roman Empire, when I was describing raiders hitting the shore I used elements of The Vikings, and when the players found themselves in a huge battle between two nations I used the great war scenes from Alexander the Great and Spartacus. As soon as I said 'have you seen Quo Vadis? The city is burning like Rome' the scene was set and it made for one of the best action sequences I think I've ever run in a game, with the collapsing buildings, panic in the streets and the struggle to get out of the city as the capital was ransacked, and the players had find the way to their tavern to get hold of the riches they had accumulated over the last few games.
These films were a great source of inspiration for me in my formative gaming years, and they still have something of an influence on me now. After Gladiator hit the screens I ran a huge Warhammer campaign with those elements, using stills from the film to get my point across, and recently I've been thinking about a Conan game and I used images from Ben Hur and The Ten Commandments to help visualise ideas. As well as being amazing movies they can really help fill out a world and can be source of amazing melodrama.