If you want the icon to display correctly on a web page, you need to edit the HTML code of the said page by specifying the path with the link tag between the file header tags. Below you can see an ...

7. Convert optional images… If you are not satisfied with the existing icons, you can create your own. You can take an existing image and convert it to the ico format.

Open the image you want to use for the folder's icon in Preview. Press Cmd+a (to select it) and then Cmd+c (to copy it). Back in the info panel, click on the icon in the top left corner and press ...