- locate the image file you want to use
- press Cmd-C (Edit:Copy)
- locate the folder you want the thumbnail applied to
- press Cmd-I (File:Get Info)
- click on the top left icon until it gets a colored outline
- press Cmd-V (Edit:Paste)
You can also copy existing icons, use the Get Info window as source.
If this doesn’t work, open the image in Preview.app, select part of or the whole image and press Cmd-C (Edit:Copy). Then proceed with step 3.
I’d go with drag’n’drop:
- open info of file(s)/folder(s) to set icon on
- locate your icon (it can be an icon file or just any file, which icon you see for that file will be the one you will set)
- drag icon and drop it on top left icon in info window
You need to create a special Folder-type icon (the usual app bundle .icns won’t work here). Img2icns will give you an option to create this type of icon, despite its name.
Customizing file and folder icons in Mac OS X presents a more involved process utilizing an .icns file which can contain several versions of an icon for display at different sizes. While less elegant than the ⌘ + C, ⌘ + I, ⌘ + V method, it’s useful in cases where scaling a single icon produces unacceptable results (such as when the 16×16 icon is an indistinct blur).
So it cannot be like thumbnail for Pictures folder in Windows? I mean, if I’ve set the display option to Thumbnails, then if a folder contains images i.e. photos, wallpapers then it gives a preview on the folder icon. I guess it’s not there in OS X or any other OS for that matter..