Here’s a quick fix for WordPress 2.X to WordPress 3.X migration:
Fatal error: Cannot redeclare wp_load_image() (… previously declared in /wp-includes/media.php) in /wp-admin/includes/image.php …)
What causes this? The primary cause is a plugin inside /wp-content/plugins folder that is incompatible with WordPress 3.0. Most likely the plugin developer defined a wordpress function inside their plugin folder, causing a function name redeclaration.
Fix? If you have the customize plugin inside /wp-content/plugins/, remove it. This was the plugin causing the fatal error for me. Login to the wp-admin control panel and re-install the plugin.
If this does not work, rename your plugins folder to plugins-backup. This should allow your WordPress to work again temporarily. Copy over the plugins folder from the fresh WordPress install /wp-content/plugins/ to your server. Move back in the plugins one-by-one from the plugins-backup folder to your new plugins folder. If you find the plugin the causes the fatal error, move it out. See if a new version of the plugin is available for download from your wp-admin panel.