First thing we need some cross hairs, unfortunately the assets store doesn't have any free ones. So, google cross hairs, try and fine one that is in the .png format. This is because the .png format supports transparency. This is important, because we want to be able to see through the cross hairs and not have surrounding colors take up screen space. You can usually tell if its transparent in the google search because if you click the image it will show a checkered white and grey background, secondly when you save the image to the desktop, you should just see the image, and no other background. If you do need to make your image transparent i would suggest the online image editor: http://www166.lunapic.com/editor/
Now import your image(ill leave that to you). Click the image and look in the Inspector tab. Under Texture Type select Texture.