Mac OS X: Customising application icons in OS X is an extremely easy process. Simple drag and drop will usually do, but for more integrated applications and system files there are a few quick techniques that can help.
Designer Jessica Hische's video demonstrates how after creating your own application icons, it's quick and easy to replace the original icons with your own.
Replacing Your Icons
For third-party icons like Chrome and Photoshop, the simplest method is to right click on your application, select "Get Info" and drag and drop your new icon (created in Photoshop) atop the default preview icon on the top left of your menu. This will immediately replace the old icon with your new one. If you're unhappy with the new icon, clicking it again in the "Get Info" field and hitting "Delete" will replace it with the default icon.
For Apple's OS X applications like iChat and Photobooth, the easiest method to replace default icons requires free application FastIcns. Opening FastIcns, you'll notice a rotating three-dimensional cube on your desktop. Drag and drop your new icon (saved as .png) onto the cube. When asked, be sure to select all available size options to create a single .icns file that will contain multiple sizes of the same icon for OS X to default to. Then by "Showing Package Contents" you should be able to navigate to the default .icns file and replace it with your new one.
Unfortunately Jessica's methods don't discuss changing Apple's default system icons like the Finder, files and folders. To remedy this you need a free download called LiteIcon. Similar to CandyBar, LiteIcon let's you swap a system icon for a custom icon via drag and drop.
Take customisation a step further by considering changes to your Dock and other OS X resources as well. Also worth noting is freemium application CandyBar which allows you to store and change 200 custom icons before charging you.