How to select all textures used by all active materials in the scene? I have a project in my hands that has one object mesh (a city block with buildings) with around 10K materials that each have a texture in them. The textures for these material slots themselves are in their own folder. 

I wanted to take a small piece of that city block (still with around 500 materials) and put it all into a neat package with only active materials and needed textures. 

Cutting the piece out of the city into its own file and With the help of Materials Utils Add-on I was able to delete the unused materials. 

But how could I round up the used textures? From that folder of some 10K images I somehow need to pick out the ones that are in active use in the materials. Any ideas?
If you save then reload your file, Blender will get rid of the unlinked (unused) images. (and Materials, by the way)

Then you can use File/External Data/Pack all into file, you should end up with a .blend that contain every image used in the scene.

If you need to keep them as separated files, you can copy your packed .blend in a different folder, then use External Data/Unpack all into files.