use Date::Calc; # For date math

my $day = some formatting to localtime...
return if $day > $end_date;

while ( $day > $start_date ) { # you'll have to either keep these in YYYYMMDD format, or do some more complicated comparison logic
   my $hopeful_filename = "images/$today";
   if (-e $hopeful_filename) {
       return $hopeful_filename;
   }
   $day = Add_Delta_Days(...); # Subtract one from the day
}