Hi Hippie,
If you have the FBX and the texture already, then it's going to come down to renaming the texture, and renaming the material name in blender (this is how Horizons will recognize that the object you're importing goes with that texture). The fbx file name doesn't really matter, but the texture does. Most (not all) textures can be a simple _BR file. Like Test_BR (and you can only import png textures by the way, from what I've seen). Then you have to make sure that the material name is named the same as the texture in blender. Whatever material name it has, rename it to Test (or whatever name you want, but it has to match the texture name). To import, you will import the fbx along with the texture file. If you get any errors, the errors will tell you what you're missing. Hope this helps!
-Kayrrie