Tutorial I found with google:
Use Paint.net or Gimp.
That tutorial is for potatochops… Is there anything that will explain it for gimp?
I think you want to check our Stickie which should have a tutorial about creating materials. And you’ll probably want better images.
You might have an easier time getting some textures from Arroway http://www.arroway-textures.com/
For the floor tile, you can check out their Tiles section. They offer average quality samples for you to use.
The ceiling could be done easily simply with the color of your choice and adding some low-opacity noise to the image.
Those pictures are horrible. You will have an absolutely hard time doing anything with them, and getting them to look even remotely good ingame with that type of noise and that low resolution.
Also, GIMP has extremely close layout to photoshop, you should be able to follow along if you know even the basics of the UI