Hey, welcome to the gang
beison!!
Nice images... I'm sure you will get tons of response to your question... from my standpoint... they look find on my screen.... [ modified: I have to agree with the others on that lens flare...possible rejection]
all the agencies are different in what they are looking for, what will catch their attention with new apps. The key, as you seem to know, is technical perfection.
istock is big on artifacting/noise.... (i know, they just shot down 6 of mine this week and I've been with them for years).
and like any other agency, it depends which way the wind is blowing on any given day as to if an image is accepted or not.
the key is...
dont give up. I was fortunate to get into IS on the first shot, purely luck of the draw.... obviously as even now they shoot me down on some in every batch. LOL
[
the following is just a joke based on my last batch uploaded to IS] However they may ask for a property release on that flag, even if you have your own personal sovereign state, it looks too much like an american flag.... and the violin, did you make it, you may have to send in a property release, and of course, i am betting you dont own that railroad?!?..... [
again, just a joke based on my experience with my last batch of rejects.... some of the old timers wouldnt believe what they wanted a property release on... LOL. it's gettting sillier and sillier by the day.... 
]
Key to survive in this biz? Dont take rejection personal. I've always said, photos are like nachos... you can always go out and make more.
Welcome again and wishing you success with IS.

=tom