Well first of all, I just tried again and the error did not reoccur. But before I posted I was able to repeat the error several times today.
To get the error initially, I had clicked "customize template", typed a new template name, select a landing image and results image from my hard disk (JPG's, both of which meet size and dimension requirements) then I clicked Save Customizations. The error page then appeared.
P.S. I used the same images when I got the error as when I did not.
I hope it should be an easy error to track down. An undefined function is named above, and even the line number is identified. If you think it may help, feel free to PM me, I'll try to duplicate it again if you can't.
Another edit: Here's a possibility. My connection may have timed out before I clicked the Save Customizations button. (I may have had spent 10 minutes or so on that page working on graphics editing.)