sum(YourImage(:))
I wonder, though, whether you really want to do that for RGB images? The sum per layer would tend to make more sense. That or converting the image to grayscale and summing the grayscale values.
Warning: your total is going to come out very different depending on whether the image is represented as floating point or uint8 or uint16. And if your image is pseudo-color, you would need to convert to RGB before you did the addition.
