I have a 1024 x 3000 image and when i import this image unity turns this into 512x1024 :S what is the right way to import an image then?
This is happening because it needs to by the power of 2. (32x32 1024x1024 etc…) Also, is it stretching? Your image settings might have it to stretch to fit certain dimensions. This may be the problem. Make sure the image dimensions is 1024x1024 and it might work!
1024*6000 might cause issues with some old GPUs. Try to rearrange your elements to fall into a square. That’s how GPUs are made to work.
Now, you could use an uncompressed format like 16bit or 32 bit (but it’ll take way too much memory) since those can work with NPOT sizes.