Make sure:
1. Your textures isn't there already.
2. Size is 128x128 pixels
3. It is saved in 24bits/pixel without compression
4. It has the same name as your ingame nick with a .tga extension (ie my head texture is called hampa.tga)
5. Note that clan prefixes are ignored so [CLAN]Toribashian gets the Toribashian.tga texture.
There have been some updates to the heads import so now you can:
1. create a head texture in BMP format. Yes!.. bmp as in mspaint
2. create a head texture in JPG format. Yes!.. jpeg as in a million other graphics programs
3. gif format is supported too
The only restriction is that the texture needs to have the dimensions 128x128 pixels.
As usual heads are imported from this link:
http://www.toribash.com/heads.php