The arithmetic mean or average is easy to implement since it's often integrated as a function in most programming languages.
However, it's not really appropriate for ratings as several 5 out of 10s would "drag" down the average whereas a single rating of, say, 8 would give a higher average.
What calculation is usually done instead on sites that let you rate pictures or people? I'm supposing that weight is given to a higher number of ratings.
I think your best bet is to ignore the outliers - ie discard the top 10% and the bottom 10% and then calculate the average
Alternatively, maybe re-think exactly what you want to do, maybe have some way of assigning weights to the voters