In this Settings window here.,.

..I suggest that the 'Thumbnail' be no longer a tickbox option but rather a one-or-the-other choice option between a new proposed feature option simply named 'File associated icon'.
This 'File associated icon' would simply use the file icon of the file from the system OS rather than a tab image preview, especially since a tab image preview can begin to look ever so ridiculous for large dimension images. Really an image/picture preview on the tab is not to my taste but an option between something else while still keeping the existing option to keep nothing on the tab would be great.
So I made this, to show what the new options would look like;

So there are 3 options 'Nothing', 'Thumbnail', 'File associated icon'
'Nothing' should be better named to 'Plain' or 'Blank'
When 'File associated icon' is enabled then the tabs would simply use the icon of the file that is designated by the system OS (which is usually the highest quality 16x16 or otherwise nearest 16x16). I made this mock up on Win XP:

This feature should work cross-platform - Windows, Mac and maybe Linux sytems too.