Ok, I started scripting a bit and found the answer myself.
This may be very useful for people who want to make pixel art games. It makes sure mipmaps are off when the game engine is started.
Go to the Text Editor window and add a new text. Call it (for example) “mipmaps_off” (without " ")
This is the very simple code you have to write:
You can execute this script and it will work! But what if you make a interactive scene or a game and you publish the blend. but you doubt everyone will turn off mipmaps in the User Preferences? They won’t see the pixel style but muddy looking textures.
To make sure this script is executed, when the game engine is, do the following:
Add an “empty object” to your scene (It could be any other object, i guess, but I’d prefer this and name the empty object properly). Select it and go to Logic in the Buttons Window panel. Add a sensor and set it to “always”. Add a conroller, set it to Python, set it to Script and in this panel you have to write in the name of your script (in this example: “mipmaps_off”). connect sensor and controller. Finished!
This will turn off mipmaps in your whole scene.