Have you tried getting feedback from the istock Critique forum? Especially your final selection I would submit it there for Peer review before uploading.

Here is an example:
http://www.istockphoto.com/forum_messages.php?threadid=330680&page=1

My favorites are the owlman, strawberrycakes and the woman in the pool with the hat. But this is my personal taste, I have no idea, if this will satisfy the reviewers.

While i like a lot of your shots, i think you can improve them by thinking about the composition more. Example is the red umbrella and the cathedral in your first subitted collage. While i love it visually, there is an issue i have with the light pole and the church not creating a good negative space. I think this is the key to improve your shots. Focus on the negative shapes.  The picture would be better if you lowered your vantage point and shot on your knees. That way the top of the light pole would in the sky and not inside the church...It would also help if the red umbrella person wasnt so close to the pole (if she moved more to the right) but this cant be helped if she isnt your model. The pool picture would be my favorite if you can clone out that distracting line in the floor.

IMO if you get it worked out with the focus and the lighting I would skip the microstock business and ask Jonathan Ross how to start in macro.

I think some of your images are really good.

A lot of those are really beautiful, I hope you get in!

I GOT REJECTED AGAIN!!!!!!!!!!!!!!!!!!!!!!! ALL 3 OF THEM...

The iStock administrators have asked that you upload new samples based on the feedback provided below. You're welcome to return in 365 days, upload some new samples and we'll re-process your application.

Comments from the iStockphoto Administrator:

At this time we regret to inform you that we did not feel the overall composition of your photography or subject matter is at the minimum level of standard for iStockphoto. Please take some time to review training materials, resources and articles provided through iStockphoto. The photographs provided in your application should be diverse in subject matter, technical ability and should be your best work. Think conceptual, creative and most important think Stock photography. Try to avoid the average eye level push the button perspective of a common subject. Try and impress us, we want to see how you stand out from the crowd.

We welcome you to return after the number of days specified and upload 3 fresh samples of your work and we will re-process your application.  Please note that you will not be able to upload new samples until this waiting period has passed.

I am thinking - maybe they want to see more images done in a studio? To prove that you understand professional lighting technique?

Did you ask for feedback on the istockforums? The admins there will be more familiar with what they are looking for.

You are very artistic. Your compositions are superb.

What got your images rejected were likely the technical aspects. I can see noise and CA in some of the images. Some over-blown areas. In your portraits, sometimes the focus (the eyes) was not sharp enough. iStock is very exact on these things.

What camera and lenses are you using?

Thank you guys for the feedback 

What camera and lenses are you using?

some of these pictures have been shot with Canon 450D (little girl portraits and etc.) so they may have some noise included, but the last 3 samples that I sent were shot with Canon 5D MarkII and Canon L lenses, they were dead sharp and completely noise free.

Am I reading that right?  365 days!!??  Please tell me you don't have to wait a whole year to upload more samples.

I gave up trying to enter IStock!!! If they don't want me, that's ok, I'll sell my works where I'm welcomed!!! 7 tries is more than enough!!!!

I am thinking - maybe they want to see more images done in a studio? To prove that you understand professional lighting technique?

I've sent Studio pictures in my previous efforts!! Even one of them actually had some special lighting technique (See the pumpkins in the file titled "Rejected" in my main post above)

Did you ask for feedback on the istockforums?

I tried but it's not possible to write a post in Istock forums before passing this sample test!!!! Maybe it has been possible before, but now it is not!!!!

There has got to be som little detail that you are missing, because your pictures are much better than mine, and I got accepted with much less hassle.

I hate to think like this but I'm from Iran and maybe they prefer not to accept new members from Iran, because Paypal and Moneybookers do not provide their service to Iranians due to silly international sanctions,  and stock agencies may have some problems with Iranians when it comes to payment!! (But there are other methods of payment that I'm using with other agencies!!!!)
I can't accept that there is not even one good image in 21 images that I've sent for them during my 7 tries!!!! or maybe I'm wrong ... but during my searches in different forums, I've found a lot of weak shots with obvious photographic faults that have been accepted recently as 3 sample submissions!!!!!

Anyway I'm thankful to you all , for your comments and kind words. It's really appreciated.
Good luck: Borna

istock has contributors from all over the globe, ages, religion...etc...I sincerly doubt your location means anything to them.

I think buyers can post in the forums. Have you tried buying a few credits so that you can post? Maybe check with support, but if it is possible, spending 10 dollars is worth it.

I have heard that istock has become more strict, you are very brave to attempt it 7 times. However, I do believe we are missing something in these rejections, maybe something only visible at 100%.

the owlman I thought had composition and lighting probs (as stated in feedback). The other two were good. I believe if one photo is rejected you do not get in The year wait after rejection is due to you applying many times? Many of your photos are soft and you need to fix your technique. I do not believe Istock gives a * what country you are from. For your application you need three photos showing your skill set. All need to be great and technically perfect. After all they are your "best" .