how will I know which display is retina so to render images with half height,width ?
This is actually a topic for later but, when u acces a page ure computer tells the page the information. So if ure on a macbook it will know. U don’t have to know this u can pre-set the code so it will react for u.
If I make Images half of their size then will it not effect the design of whole web page ?
Yes it will but u can set several values for diffrent sizes if u like to know more about this subject take a dive in responcive web design