// does not run yet.
var downloadList = new List<IObservable<byte[]>>(); 
for (int i = 0; i < 10; i++)
{
 if (true) // Resouces.Load == null...
 {
 var downloadAction = ObservableWWW.GetAndGetBytes("http://aaa." + i + ".png")
 .Do(byteArray => // Do(sideeffect only) or Select(return value)
 {
 // var tex =
 });
 downloadList.Add(downloadAction);
 }
}
// 1. sequential request, does not use result
Observable.Concat(downloadList).LastOrDefault().Subscribe();
// 2. sequential request, use result values
Observable.Concat(downloadList).ToArray().Subscribe();
// 3. concurrent request
Observable.WhenAll(downloadList).Subscribe();
// 4. choose concurrent count
Observable.Merge(downloadList, maxConcurrent: 3).Subscribe();

IObservable<Unit> SequentialLoad()
{
 return Observable.FromCoroutine<Unit>(observer => SequentialLoadCore(observer));
}
IEnumerator SequentialLoadCore(IObserver<Unit> observer)
{
 for (int i = 0; i < 10; i++)
 {
 // ToYieldInstruction can await observbale.
 var request = ObservableWWW.GetAndGetBytes("url").ToYieldInstruction();
 yield return request;
 // var tex = new Tex2D(request.Result, ....);
 }
 observer.OnNext(Unit.Default); // push Unit or all buffer result.
 observer.OnCompleted();
}