The Epson Workforce 840 do support scanning on Mountain Lion via Apple Image Capture.
You may find the steps to configure the scanner by following the ICA Scanner Driver section below:
On a blue moon, if the Workforce 840 sits on for two days with Image Capture open also, you may run into a message saying Image Capture is already in use. I turn the printer off then on. Image Capture is automatically closed in the process. I then open Image Capture and all is well again. It happens rarely but it is a quirk.