just add a set texture node above your "load fbx" node. As you will see, the settexture node has two inputs. One for the mesh(es) and one for the image.
Now you can add a a "load image"-op and connect that one to the "settexture"-op as well. And voila.
For basic stuff like this (and more complex stuff as well) just check other projects that have features like these. And look at the op-graphs of those projects to learn how stuff works. That how I do it anyway.