A tutorial that I wrote for GTAGaming describing how to install this same mod:
1. Download and extract the mod. You will see three PNG files, which are the textures for turning Niko into The Joker.
2. Open up SparkIV, and click Open. Browse to GTA IV\pc\models\cdimages\playerped.rpf and open the file.
3. SparkIV will now display a long list of various .wdr and .wtd files. These are all the textures used to give Niko his skin and clothes.
4. To install the Joker mod, you will need to replace the images inside these texture archives. First, let's install the Joker's hair...
5. Find and click hair_diff_000_a_uni.wtd inside SparkIV. Next, press Edit in the program's menu. This will open up another window where you should see an image of Niko's hair (or something that looks like it). Next, press Import Texture from the top menu, and select the hair_diff_000_a_uni.png file that you downloaded with the Joker mod. When you see the new hair image appear in SparkIV, click Save and Close in the top right hand corner of the menu.
6. Installing the face textures is a similar process - find head_diff_000_a_whi.wtd inside SparkIV and select Edit. Select Import Texture and browse to the Joker mod's head_diff_000_a_whi.png. When the new image appears, press Save and Close.
7. And finally, repeat the process one more time for the Joker's suit - find uppr_diff_012_a_uni.wtd, edit it, and import the uppr_diff_012_a_uni.png texture then save and close.
8. Once you've installed all the new textures, click Save in the SparkIV menu and close the program.