Texturelib is a library of quality high resolution textures. Free for personal and commercial use.
They are free for commercial and non-commercial use with the only limitation: you are not allowed to sell or distribute original or slightly modified images.
You can't redistribute the image just changing some colors...
I think we are misunderstanding the concept of "heavily modified", we just have to use it normally.
That restriction is intended to avoid publishing the same resource into another site, he wants to get users for his website, so we can't distribute the same texture.
But if you are texturing, you Must modify those textures, you must at least make them seamless, change a the light or make them dirtier, etc. EG: if you are using a door maybe you should merge it with a wall and add a little of moss into it. That kind of stuff. Use the images as you should. Be reasonable.
So the result of the distributable files would be: 3d.mesh + final_texture.png
That's my impression, I'll try to reach him to clarify this point.