I want to show same thumbnail in all my blogs. So in every post I have to edit its html by img style="display:none;" which I dont like. so is there any option that same thumbnail should appear in all post automatically without editing their html?
You may be able to edit the template, so that that the desired image with img style="display:none;" applied is included immediately after your post header lines, in the post-body content div but before the actual content. Howerver I've not tried this, and don't know whether Blogger would pick up the thumbnail from an image outside of the post-body content or not.
Other than that, what you want is not possible in Blogger.