Theme Problem with Posts

I’m using the Reaction theme from Theme Forest and get this error whenever I add a post:

Catchable fatal error: Object of class WP_Error could not be converted to string in …/wp-content/themes/reaction-2.0/functions/image_resize.php on line 118

Posts display properly on the top level page, but linking from that or any other link to the post itself results in the above error message.

Here is the code in question:

// no cache files – let’s finally resize it
$new_img_path = image_resize( $file_path, $width, $height, $crop );
$new_img_size = getimagesize( $new_img_path );
$new_img = str_replace( basename( $image_src[0] ), basename( $new_img_path ), $image_src[0] );

Any ideas how I can fix this?