Declare $wp_version;

[nickbuddendotcom]

At least on my local server, version_compare() doesn't work if I don't declare global $wp_version.

[MatthewRuddy]

If you've just included the file containing the resizing code (such as in your theme's functions.php file), it should be fine. Provided the code isn't within a function, class, etc, it should be in the global namespace and not require any declaring of $wp_version.

[aristath]

I've had that error too on one of my installations and declaring $wp_version can't do any harm...
Better safe than sorry!