Problem uploading custom image header

I'm getting this problem with the Nelo theme.

I've installed the multi site plugin and cloned the site so that it's 3 different domains/sites. I had uploaded a custom image header on the first page without errors. I then split the site into 3 and I'm now trying to upload custom headers on the other 2.

This is the error I get:

Warning: strpos() [function.strpos]: Empty delimiter in /usr/home/qlof/sportbloggar/wp-includes/wpmu-functions.php on line 1659

And that line is:
if( strpos( $ext_pattern, $ext ) !== false ) {

What is ext_pattern? Is this simply an empty setting?