Even though I have the library imported Java doesn't recognize the function. If I call the function over the library directly it's working fine.
Like that it's not working:
import org.lwjgl.stb.STBImage;
ByteBuffer data = stbi_load(filename, width, height, comp, 4);
That just works fine:
ByteBuffer data = org.lwjgl.stb.STBImage.stbi_load(filename, width, height, comp, 4);
You are importing a class, so you should refer to the method via its class:
ByteBuffer data = STBImage.stbi_load(filename, width, height, comp, 4);
Alternatively, if you want to call the method without the class name, you should statically import the method:
import static org.lwjgl.stb.STBImage.stbi_load;
And, of course, you could use a wildcard:
import static org.lwjgl.stb.STBImage.*;