Shorten JavaScript Code due to duplicate variables without polluting global namespace
If those variables are constant (ie, they don't change between invocations of of the functions), you can put the functions and variables into a closure:

(function() {
    //FIRST FUNCTION
    var imageSum = snapshots.total_entries; //Inner Box
    var dl = $j('#snapshots')[0]; //Outer Box
    ...
    window.snapshotScroll = function (amount, refresh) {
        ...
    } // end snapshotScroll();

    //SECOND FUNCTION
    window.snapshotCheck = function (){ // Check if snapshot is in view
        ...
    } // end snapshotCheck();
})();