There's a huge body of writing on visual theory and image meaning/manipulation. Search these for relevant articles and books.
ARTBIBLIOGRAPHIES MODERN - Major journal index address modern visual studies. For gaming, use both "games" and "gaming" as search words.
Oxford Art Online - Oxford's combined art databases, searchable uniformly or individually. Includes techniques, media, subjects
A Guide to the Web's Free Image Resources (2013 Atlantic article) - still a nice survey.
VADS - Image Index - a service of the Library & Student Services department of the University for the Creative Arts.
Reanimation Library - copyright-free books heavy on images and visual content
Sometimes you just need to see a ton of images to find the perfect one. Books have the advantage over databases in that their subjects are more limited than a broad web search and their source/owner more reliably documented. Fun, disparate image browsing includes:
Graphis design annual [serial]. Library Service Center: NC997.A1 G73 LSC - art journal of art/design - commercially centered, but still very creative
Art directors annual [serial]. Library Service Center NC997.A1 A69 - Annual summary of awarding winning designs in the advertising field - includes print, video and internet
All-American ads (series). NC998.5.A1 A155 A decade-by-decade tour through commercial art, each decade a different volume (the 50's, the 60's, etc.)