Now if how to use images with @1x, @2x & @3x.

Lets say you have an imageView in with size of 100W x 100H, to display images properly on all the devices with appropriate resolution without stretching or pixelation your image, one need to create 3 set of images as below according to device.

• iPad 2, iPad mini supports @1x images, required image of size 100x100(WxH)
• iPad, iPad mini supports @2x images, required image of size 200x200(WxH)
• iPhone 4S, 5 & 6 supports @2x images, required image of size 200x200(WxH)
• iPhone 6P supports @3x images, required image of size 300x300(WxH)

Though your UIImageView size is 100x100 for all the devices, but image displayed in different devices is different according to their resolutions.

Following size Table will give you more clarification, read more about it in apple documentation here