UBB.threads PHP Forum Software Community Forums UBB.threads 7 Installation and Upgrade Support Fatal error: Call to undefined function: resize_image

When I try to upload a picture using the Image manager, I get this error:

Fatal error: Call to undefined function: resize_image_() in /home/content/c/l/a/clg/html/forums/libs/image.inc.php on line 66

I suspect it has to do with GD2, but I am running PHP 4.x on my server, so GD2 should be installed by default, right?

Any help appreciated.