Is this a good strategy to optimise web images for iphone?
I think you are on the right track of optimizing images for mobile devices, but I think that the way you are going about it won't scale very well. If you just target iPhones with resized images, you're going to be supporting 5 different sizes, iPhone 6+, iPhone 6, iPhone 5/5S, iPhone 4/4S, plus the original, which isn't that bad. 

But what if you want to support Android phones at the same level, having an image for every device made? You're going to end up with dozens of different image sizes. 

Android screen resoultions

Here is another link with a larger list of device sizes.

mydevice.io common smartphone sizes


Not only will you have dozens of different devices, but your naming scheme won't work that well, as some resolutions are used for multiple displays, leaving you with duplicates of images. 

A better way to optimize for mobile is to target screen resolutions instead of devices. For example, you could target a set of resolutions like 


  xxxhdpi: 1280x1920 px
  xxhdpi: 960x1600 px
  xhdpi: 640x960 px
  hdpi: 480x800 px
  mdpi: 320x480 px
  ldpi: 240x320 px


With this method, you can support all mobile devices with just 6 different sizes, plus, when a new device is launched, you won't need to resize every image on your site for the new device. 

This is all assuming you are just talking about resizing your image assets. If you are going to be redoing your layout for every device, you would be much, much better creating a responsive site that scales for mobile devices.