Why would you want to rebuild your thumbnails? Wordpress allows you to change the size of thumbnails. This way, you can make the size of thumbnails fit the design of your website. When you change the size to fit for a new theme, all future photos you are going to upload will have this new size. Your old thumbnails won’t be resized. That’s where this plugin comes into action. After changing the image sizes, you can rebuild all thumbnails. But instead of telling the server to recreate all thumbnails at once, they are rebuilt one after another. Rebuilding thumbnails for one photo won’t take all too long, so you won’t run into any script timeouts. Note that you still have to wait until all thumbnails have been rebuilt. If you close the page before the task is completed, you have to start all over again.

You can download the plugin on it’s homepage.

Unfortunately there is no option to change this value for existings blogs, but you can you change it directly in the database:

UPDATE `#YOURDATABASENAME#`.`wp_sitemeta` SET `meta_value` = '32000'
 WHERE (`wp_sitemeta`.`meta_value` = 1500)
 AND (`wp_sitemeta`.`meta_key` LIKE "fileupload_maxk");