You can normalize your image wrt itself. This would be taking an image and for each channel: summing up all its pixels, compute mean and std and normalizing .
Yoy can normalize an image wrt the dataset it comes from. This would be taking the whole dataset and  for each channel of each image: summing up all its pixels. Once you have the total sum for the whole dataset, just divide by the amount of pixels (of the whole dataset). Obviously this may be very slow or not feasible for a very big dataset in which case you can take a representative subset or to use some maths