Hi,
I just wanted to thank again to all for your kind and helpful suggestions!!
I submitted three new test photos to iStock and they were all approved 
Thanks,
Z

The composition of Z3 is altered, since I didn't want to resize the image, in the original photo the object is not positioned so central.
I must confess, some of your cropping opinions are a bit confusing (too much or too little background ), but otherwise thank you all so much for the critique - no other way to improve

Hi all,
I am trying to get an iStock account. I have sent them the three test photos and all three were rejected with the same explanation: "We could not find a clear center focal point for this file."
The files are these:
http://www.flickr.com/photos/22450069@N04/?saved=1
(the one with the sand background is a bit cropped from the left and top, but the resolution is unchanged)
I am not a professional photographer, so I'd be very grateful if someone could please post a comment of these photos.
Where is the lack of focal point seen?
What are the other problems of these photos?
Are they good enough that I should keep trying with microstock or not?
How can improve my work?
Of course I'm not taking pictures to make money from them, but I just thought I could give it a try and now I want to learn more  ...
Thanks!!