Even though PneumaticCraft adds images for IGW-Mod in the assets/igwmod/textures/ path, this totally isn't necessary and is not recommended. In the early stages of IGWMod it was. Now when referencing images you can use the image command like [image{<x>, <y>, <size>, <resourcelocation>}], with the <resourcelocation> being for example pneumaticcraft:textures/wiki/testImg.png. So it can be prefixed with any mod id, as it's an ordinary resource location.