Now I wonder what if I use one of your textures in a project and then I decide to give away (for free) the model to a friend or post a download link for it: some formats like .obj have to come with their .mtl and image based textures... is it allowed or not? is it sufficient to credit your site or ...

I have created site with my own collection of textures - http://www.texturer.com. I'm interested to hear your opinion about this one.
The textures are free for use in any situation by anyone.

Serg