It couldn't be easier, Program Settings, Exclusions, Add, e.g. G:\ImageFolder\*.jpg or *.bmp or just * after the folder location, it is better to use the wildcard with a file type so any other files that might end up in the folder will get scaned.
So no con at all it is built into avast to exclude files or folders, etc.
Also see the avast Help file, Settings, Exclusions Page, a really helpful document.