setting textures for ipad/iphone

I am developing a game for iphone/ipad Retina/nonRetina display. I am a noob so don't know what the scaling situation is on Unity. In cocos2d i used to place 4 types of images (coresponding to all resolutions). How can i set in my game these images to be loaded for each device?

i.e. in cocos2d the convention is, that for an image "ima.jpg", there will be 4 images "ima.jpg", "ima-hd.jpg","ima-ipad.jpg","ima-ipadhd.jpg" coresponding for all the devices. Is there such a method in Unity so that images may be selected (even if i load them through code, even if i manimulate them through the unity editor) so that the appropriate image is selected when loading the image lets say on a GUITexture ?