These two functions (psImageStats and psImageBackground) are different in small but important ways. Both are used in the code base. They both use psVectorStats for the low-level calculation (though psImageBackground supports an additional statistic PS_STAT_ROBUST_QUARTILE which psVectorStats does not support -- it is also probably not interesting to keep).
They probably should be used in different cases (eg, size of image, desired speed and accuracy, etc).
It is probably still worth the effort to merge them, if only to make the usage clear
