In the olden days, one of the marks of a serious Mac user was whether or not they had customized icons. There were not a whole lot of sources for System 7 icons (or 8, or 9...), so not only did you have to be able to track them down but you also had to have the skill to apply them.
It’s gotten easier—today, it's less complex to bend icons to your will. Whether you prefer the previous version of an icon, or you’re a big fan of something and you want to make sure it shows up all over your computer, it’s not as complex to change icons these days.
First up, icons in OS X can be anything you want. Do you want famous race cars to represent all of your folders? No problem. Have a research project where each file should be represented by celebrities that died young? OK, whatever floats your boat. There is probably an icon set out there to satisfy even the most eccentric of tastes.
You can also make your own icons, but that's a subject for another article.
Once you've found an icon you'd like to use, here's how to use it on your Mac. These instructions work in all recent versions of OS X, including Yosemite, Mavericks, Mountain Lion, Lion, and even earlier.
First, find the image you want to use and open it in Preview. Go to Edit > Select All, then Edit > Copy. Now that image is on the clipboard.
Yeah, Kelly's using Doctor Who icons. Shocking. From Iconfactory.
Next, switch to the Finder and click once on the folder/app/file you want to change. Go to File > Get Info and the info panel will pop up, displaying the icon in the top left corner of the panel. My example is my hard drive.
Greetings, plain old hard drive icon. Your time is short.
Click on the small icon in the top left, and then go to Edit > Paste. Now your icon will be updated!
This works on folders or files as I mentioned, and also drives, so if you have your icons set to view on your desktop this can help improve your view.
Gotta be careful opening this. Spoilers...
But where to find those icons? If you want sources, there are a lot to choose from. Notable is the icon section of the Iconfactory. They’ve been at this for nearly two decades, and a lot of their icon sets have coordinating desktop pictures. I’ve been downloading their freeware icons and wallpapers for almost that long, and all their items are fantastic.
You can get a set custom-designed for Yosemite by Sebastiaan de With called Muir, named for John Muir, known as the Father of National Parks. Replacement icons are available from Dribbble or DeviantArt for free. You can also get the Glacier icons from Yosemite, or you can get the Sketch files for USD$5 and customize the set to your liking.