You may like:

use strict;
use warnings;
use 5.010;

printf "%s -> %s\n", $_, next_file($_)
    for qw/a.png z.jpg az.png zaz.png ott.jpg zzz.tiff oi.png/;


sub next_file {
    my ($previous) = @_;
    my ($name, $ext) = $previous =~ /(.*)\.(.*)/;

    if ($name !~ tr/ilo/jmp/) {
        $name = reverse $name;

        for my $chIdx (0 .. length($name) - 1) {
            substr($name, $chIdx, 1) =~
                tr/abcdefghjkmnpqrstuvwxy/bcdefghjkmnpqrstuvwxyz/
                and last;
            substr($name, $chIdx, 1) = 'a';
        }

        $name = reverse $name;
        $name = 'a' x (1 + length $name) if $name =~ /^a+$/;
    }

    return "$name.$ext";
}
[download]

Prints:

a.png -> b.png
z.jpg -> aa.jpg
az.png -> ba.png
zaz.png -> zba.png
ott.jpg -> ptt.jpg
zzz.tiff -> aaaa.tiff
oi.png -> pj.png
[download]