do once:
{def $nrimages=fetch( 'content', 'tree_count',
                   hash( 'parent_node_id', 62,
                          'class_filter_type', 'include',
                          'class_filter_array', array('image')
                          ) )}

do for every random image:                          
{def $randomimage=fetch( 'content', 'tree',
                   hash( 'parent_node_id', 62,
                          'class_filter_type', 'include',
                          'class_filter_array', array('image')
                          'offset', rand(0, dec($nrimages)),
                          'limit', 1
                          ) )}