I like this idea as I have thought of this feature many times personally. Althought, there's one thing that restraints me to do it. The webmaster can already modify his width and height from his ACP's configuration page. Meaning, what if the webmaster chosed to add big sizes for his users ? By choosing big sizes, even by a max number of 3 from your index page, it will definitely affect your site header since each pictures will show as different sizes on width and height.
Would you be willing to take that chance ?