Go to your dashboard, Settings->Media, set the Thumbnail Size to 100×100 (default 150×150). And use Regenerate Thumbnail Plugin :)) You can use 100×100 thumbnails after that.

Thanks!

I think we have a simple soluton :) Can you add this in the array area;

‘orderby’ => ‘menu_order’,
‘order’ => ‘ASC’, //If reverse use DESC

Also i don’t know any solution about space problem in the link. If i find the solution i will inform you :)

The problem is that the first image it grab it was the first from the array, but your previous function was the first image from the post content.

So when I change numberposts => ‘-1′ I can see list of URL image with lots of space delimiter and the problem is that the first image store as last.

And I cant just simply change the index from $thumbnail[0] to any other number it will just give me wrong output like http://mydain.com

Thanks for your reply. 
First Solution. You can use Renegerate Thumbnail Plugin for wordpress. Set the thumbnail size from WordPress Dashboard Media Settings and use this plugin. You can download from wordpress.org http://wordpress.org/extend/plugins/regenerate-thumbnails/

And second solution :) 
Look code :) You can see echo $thumbnail[0]; :))

I have two question.
Where can I resize the generated thumbnail? 

And How to make it grab the first image like your previous function you had?