An email should already be being sent to your moderators. As long as the image field is marked to require approval it will send a notification to moderators on upload.
The workflow module shows notification about canvas images, but this is memory and processing power consuming as you mention on the module setup. Our site has more than 20000 members and we are afraid of enabling "CB Plugins integration" in workflow module.
I'm not sure what performance issues you're concerned about. We've over 500,000 registered users and CB Workflows Module works just fine to notify us of pending approvals.