Efficient ways to optimize the downloading and unloading of large textures?

Hi guys I am working on a project that downloads lots of textures, many of them for a cube map, places them , and then once the player changes their options we unload and the textures and begin loading our new set of images . I wanted to know is their a more efficient way that I can be doing this I have recently begun to encounter some memory issues on both android and iOS machines .

Here is an example of unloading one group of our images :

 public void UnloadTextures(){

     Resources.UnloadAsset(leftPano);
     Resources.UnloadAsset(rightPano);
 }


 private void UnloadTextures(Renderer pano){

     Material[] mats = pano.materials;

     for(int i = 0; i < mats.Length; i++) {

         Texture2D tempTex = mats [i].mainTexture as Texture2D;

         DestroyImmediate (tempTex,true);
     }

     Resources.UnloadUnusedAssets();

 }

Here is a second example of unloading our texture in the project :

         Texture2D leftTemp = leftShutter.texture as Texture2D;
         Texture2D rightTemp = rightShutter.texture as Texture2D;

         leftImage.texture = (Texture)PortalCache.LoadImageFromCache(downloader.DownloadedImageLibrary.leftSpinPaths[index]) as Texture;
         leftShutter.SetNativeSize();
         rightImage.texture = (Texture)PortalCache.LoadImageFromCache(downloader.DownloadedImageLibrary.rightSpinPaths[index]) as Texture;
         rightShutter.SetNativeSize();

         Resources.UnloadAsset(leftTemp);
         Resources.UnloadAsset(rightTemp);

Is there anything I can be doing that is more efficient in terms of unloading these textures? As I mentioned I am also downloading them from web , So perhaps there is an efficient way to manage downloading of textures from the web ?