Most used file extensions on Mac are icns files, png files and icon files (empty folders). Why these different extensions? It depends of what you want to do and which way you use to change your icons. When it comes to free release, some designers include these different icon files while others only include one (maybe two) file extension.
You can use an icon manager app like CandyBar or IconBox to simply convert your files from Icns, PNG or Icon files but here is how to convert them without any costs.
PNG and Icns files
PNG are the most common files. It’s a simple image file which contain only one-sized icon (most often you’ll have different PNG files which means different sizes). They are useful because they are not system dependent files, many non-Mac designers use PNG.
While Icns are Apple icon files, most used by Mac designers. These files contains a number of images in sizes up to 512×512 pixels depending of the artist. Professional icon designers always includes different sizes to keep their icons crisp in large or small sizes.
How to convert PNG or icns?
First app is the well-known Preview.app. Preview is a very handy app for designers when it comes to icons. You can export from PNG to Icns or from Icns to PNG and it’s pretty simple. You just have to open your PNG file in Preview, then save as Icns! Be aware that due to the one-zised format of the PNG file, you’ll have a one-sized icon which means all sizes will be scaled up or down.
To convert your Icns to PNG, you just need to open your Icns with Preview.app. Select the size you want to export in the sidebar, then save as PNG.
Then we have Icon Composer, most useful app for Icon designers. You can find Icon Composer in your Xcode Tools (Snow Leopard Installation DVD in Optional Installs).
Simply drag all PNG files into the box corresponding to their size, or just apply the same PNG (if you only have one size) to all. Then, save as Icns file. Remember, PNG files are one-sized image. So they will be scaled up or down to display correctly.
If you are too lazy to find your Snow Leopard Installation DVD, you can download Img2icns.
You can convert your image files to icns files but it does not provide support for different sizes, having the same effect as the Icon Composer option “Copy to all smaller sizes”, all sizes will be scaled up or down.
Or you can grab Hobiconer, an Icon Composer like with some handy options like export in ico and iContainer (icon container file used by CandyBar and IconBox), different backgrounds in preview and more sizes (24, 64).
Icon files (empty folder with the icon) are such specific files but can be useful to change your icons (it’s simple with Cmd+i, while it don’t work with Icns or PNG without copying with Preview.app). You can’t convert them without an icon manager app like CandyBar or IconBox using their export options, not a free solution but the only way we know.
Ico files are Windows icon files. You’ll find some ico files on Iconpaper because all designers are not Mac users and Windows also need icons.
To convert them for your beloved Mac, you can use Preview.app (open your ico file then save as icns) or this website which also can convert Ico files to Icns files in two easy steps (it can convert PNG too but it does not provide size support like Img2icns).
Note that if you want to convert an icon for Windows and create an ico file, you’ll have to use Hobiconer and export to ico.
That’s it, that’s all!
On Mac, icon fanatics and designers are at home and Apple got it. Icon conversions are easy on Mac, some easy steps and you’re done. With Preview.app, each Mac user has a handy multi-tool to convert their icons from ico, PNG to icns or from icns, ico to PNG.
I know there are other free apps to convert icons but I chose the most relevant and best apps around. Convert your icons and start customizing now!
PS: Remember to use all different sizes when you can for better dock icons.