bug#8453: enhancement to image mode

[Chong Yidong]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> With the help of Drew Adams on gnu.emacs.help, I added the following
>> to image-mode, which allows one to visit the following or previous
>> image, which is useful if images are in a series, such as in pages of
>> books.  If this is deemed useful, I would like to have this added to
>> emacs.
>
> This looks very handy, indeed.  I'd recommend to slightly change the
> code so it just looks for the first filename greater than the current
> name (so it also works in the odd case where the current file is not
> in the directory any more).

Done, with some substantial modifications.