It seems to me, I found the reason of such behaviour.
The thumb preview for newly added image was generated incorrectly (the colors just mashed, unfortunately I cannot attach the snap shot to show it to you).
Therefore I couldn't find the image, though it was actually present in the gallery.
The source image is as follows: http://www.educopark.com/Users/Uploads/2d411e4e-6096-43f2-bfbb-9a88e1cb550d/ship-change-direction.jpg
And if I upload the same image on your demo page (http://www.cutesoft.net/example/general.aspx), I get the same wrong thumb image!
Any ideas why the thumb image preview is not properly generated?
Sincerely,
Andrii Kovalchuk