Once you’ve installed the Arkham Horror Module, OCTGN will automatically generate card images as needed using the card data. These ‘proxy’ images are perfectly playable right out of the box, albeit they won’t have any of the beautiful artwork that this game is known for.
This is where Image Packs come in. These .o8c files contain scanned images of every card. They are organized by Expansion / Mythos Pack, so every physical release from FFG will eventually have a .o8c counterpart of card images (though separate Mythos Packs will eventually be combined into a single Cycle pack for easy downloading). With respect to FFG, there will be a 6-month delay between when a product is released and when its corresponding Image Pack will become available.
Image Packs must be installed manually within OCTGN. To do this, download the desired packs, then load up OCTGN. Go to the Games Manager tab, then click on the Add Image Packs button. Browse to your downloaded packs, select them and hit OK. You will need to repeat this process any time you download a new Image Pack. And now, onto the packs themselves!