How can I change an icon for Mac's dock bar?

I have a set of icons, and I've changed icons for folders before with no problems, but this set is saved as .png. It's for the dock bar.



When I right-click the icon and try to copy the small folder icon/the new picture, it's only shown as a gif/jpg/etc preview, not the actual icon image itself.



I downloaded it from deviantart. No one else has had any problems with it, so what format/what do I do/change to be able to see the icon in it's 'See info' preview to copy over to the folder I wanted to change in the first place?



I will ignore all off-topic, junk, spam, flames, etc.How can I change an icon for Mac%26039;s dock bar?
Try using this site, it seems helpful.



http://interfacelift.com/icons-mac/faq.php1



Good luck!