A couple of things I would check on:
First, check to make sure you have a directory in the assets folder called ’galleries’ and that it has full writable permissions.
Second, make sure that the GD PHP extension is installed. View the phpinfo() from Reports->System Info to find out.
Lastly, if GD is installed, check the error log on your server. I had a client where their server was reporting a ’gd-jpeg: JPEG library reports unrecoverable error’ line in the logs. Not sure what the hosting provider did but if I had to guess they likely just reinstalled the GD extension.
Hope these suggestions help.