MobyGames - a long running site with thousands of games, with excellent amounts of detail about releases, and often will have screenshots from differing versions of a game. The most comprehensive game information archive that I know of.
Imgur - Neurotech - Galleries of LucasArts adventure game backgrounds, minus characters. Great if you want to see the background art without characters in the way - though be aware that the GIF format means some of the colours are slightly different. Also has scenery from Infinity Engine games, if that's your kind of thing.
Sierra Wallpaper - An excellent resource for Sierra developed and published games, with dozens of screenshots per game, showcasing hundreds of scenes. The site allows you to select a resolution to download the images at, making it an excellent resource if you want to use these old games to spruce up your desktop.
Adventure Rooms - Screenshots of many adventure games, resized to correct aspect ratio, with nearest neighbor up-sizing. A great range of games that doesn't focus on any particular publisher, and especially handy to show the older games at the proportions they were designed to be seen.
Adventure Games Backgrounds - Many screenshots from adventure games, including some less known ones, and Sierra games without characters in some examples. The images aren't pixel perfect due to the manner in which they were captured and saved, though, so keep this in mind.
If you're looking to browse through some adventure game art, these are excellent resources. I really appreciate being able to turn to these archives to find a specific shot, it minimizes the need to play through each game to find the scene I want to capture. Feel free to let me know of any others I've missed in the comments section.