The items that are modified in a turning table implement ILatheItem, so you'll probably want to return the resource location of the image you want to use via ILatheItem#getTexture(ItemStack).
If you want to make a custom item, that should be modifiable in the turning table, you need to implement ILatheItem in your item class.
If you want to use a custom variant of an existing item, you can just inject the tooltip name into the language registry. The icons are dynamically rendered.
If you want to change the icon of a variant of an existing item, that is currently not possible.
If I completely misunderstood you, please let me know.
Thanks for all your above reply, to clarify, I'm not a mod maker (yet), I'm just using minetweaker and custom items mod.
If you want to use a custom variant of an existing item, you can just inject the tooltip name into the language registry. The icons are dynamically rendered.
Yes, it is exactly what I want to do. If item icons are rendered dynamically, seems that it is possible to add variant of existing iron turning blank and just use minetweaker addTooltip function.
The items are rendered dynamically, but only on an individual item basis. If you extended the Iron Turning Blank itself for example, it would still be an iron turning blank so would still use it's texture, which you wouldn't be able to change unless you could override the texture method (which would probably be way beyond the scope of your average custom item mod). The tooltip it has you can change easily as that's designed on a per meta basis, but the texture isn't.
Yup. That's what you can do 
If you wanted I could probably make a small mod to allow custom turning blanks. (It would be even easier if you like the custom item mod you're using already so that the format is the same).
You could inject your name to the language registry instead of adding it to the itemstack via minetweaker. That would resolve the problem with the blank tooltip still being there. Look at the IC2 language file to see, how it needs to be named.
