Every mod (that is Minecraft Forge's way of talking about extensions) has its own sub-space in the wiki. Every mod's sub-space is located within the root space :mods:. Every image of a block or item is named exactly like its description page in the wiki. And is provided as PNG.
Vanilla diamond item link will be:
As noted above you should have diamond.png uploaded as mods:minecraft:diamond.png and a corresponding page located at mods:minecraft:diamond.
Configuration and Settings
You can set the following:
- Show icon or not
- Size of icon
- Make item link bold or not