Downloading / handling Resouces from server

In our game we have some .png image file with some version (say 1.0) that are stored on the server end. for 1st time we have to download entire image and store it in the resource folder through coding. Later on we will use this image.
Every time the game is launched later, we will check the server for the any updated resource and download the newer one if any.

What should be the best approach of doing this ? Also I am unable to create asset bundle of png files. Any help regarding this would be appreciated.

Request that the server gives you a list of all changed images since the last time the game was launched. So, you’ll need to store this launch time, possibly in a playerpref. Once the server has told you the names of the images which have changed, simply download them and write them to device file system.