Already to contradict myself...

I was assuming you had already played with the "Combine Textures" option in SketchUp. It does approximately exactly what you want. It has limitations though. It only works on co-planar faces though. So on a small model, it would be possible to rotate all faces, or use Jim Folt'z flatten script to flatten all faces, then combine the material into a single jpeg. That would be veryt tedious and not entirely useful on very large models though. But I suppose it could be exactly what you want if you only need it for small amounts of work.

To use it, select to faces that are coplanar, and one must already have an image material on it. Select them both (or more than just 2) and right click, and choose "Combine Textures". That will put them into a single jpg. Then you can right click on one of the faces and choose "Texture > Edit Texture Material" and it will open the jpg into your defaul image editor (You can specify that in your SketchUp preferences to use whatever image editor you have on your computer). So that is an option if you want to go through that process 😄

Chris