Acorn Image Browser
There are 2 ways to browse images files through Acorn. The first method is to go to File ▸ Open Folder (⇧⌘O). Choose either a folder that contains images, or an application. The second method is to hold ⌥ ⌘ (option, command) and drag your file or the icon of the application and drop it on top of the Acorn icon in your Dock. Either way, the image browser will appear in Acorn with all the images contained within the folder or application.
The file browser window will give you a warning if you open up a folder and it can't find any images in it.