Since many people asked me this, i though it would be easier to just create a topic about it instead of replying to 10 people answering the same question.

There is basicly 2 things you need:

a) Some software to modify images like photoshp or Gimp.
b) BLP2PNG

Steps

Most people i figured want to change the look of their actionbarbuttons, however you can use this method to change nearly any graphics you see in World of Warcraft.

1)To start, the easiest thing is to search the icon of you skill @ http://www.wowwiki.com/Portal:Main.
Lets take 'Charge' as an example. You search for 'Charge' and come up with http://www.wowwiki.com/Charge. Now on the top right corner, you have the charge-icon.

Click on it and click on it again on the next page. You end up with http://images2.wikia.nocookie.net/__...ior_charge.png. Save it on your harddisc. The good thing at wowwiki is that the icons are saved with the same name trhe game handles them, so remember >Ability_warrior_charge<.

2)Now you can open the file with your image software and modify it.

3)Save it under the same name as on wowwiki and in .png format.

4)Blp2Png is only 1 file. Drag your Png file onto the exe file to convert it to blp.

5) Go to your World of Warcraft/Interface folder.
Create a folder named 'Icons'.

6)Put the blp files you created in this Folder.

7)Enjoy

Please note, that many icons are used for multiple talents/skills/whatever and this method changes them globally.
If you want to change graphics other then icons, its easiest to downlaod an mpq editor(http://www.zezula.net/en/mpq/download.html), load your locale wow mpq and extract the graphics then convert em to png, modify them, reconvert them to blp and put them in the appropriat folder(which you can find out by checking out the structure in the mpq file; your interface folder has the same structure).

Greets Miiru