Fix error when making default image

Without the $image variable you will get a warning similar to the following. This image name is passed by the make default checkbox but was not caught in the code.

Warning: getimagesize(E:/xampp/htdocs/auction_dev/uploaded/55t54tre3kriv568jtq7b96635/): failed to open stream: No such file or directory in E:\xampp\htdocs\auction_dev\upldgallery.php on line 201

Warning: Division by zero in E:\xampp\htdocs\auction_dev\upldgallery.php on line 204