Using "Thumbnails" to index downloadable images
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I am envisaging a situation where your site contains a number of large
image files, and SOME of the visitors would like to look at or download
SOME of those images; but it would be antisocial to force ALL visitors
to have to fetch/download ALL of those large image files.

If the HTML has a standard image tag <IMG SRC="huge.jpeg">, then that
image will always be fetched and rendered, unless the visitor has
turned off graphics.

If that is replaced by a hyperlink
<A HREF="huge.jpeg">Artistic masterpiece</A>,
the image will not be fetched/downloaded unless/until the visitor clicks
on the link. If/when they do, the image file will be fetched, and then
depending on their options configuration, their browser will either:
(a) present a "Save as" dialog box (or equivalent if non-Acorn);  or
(b) fire up ChangeFSI and display the image in a separate window.

If you have several such image files, it can look nicer if each link
is shown as a "thumbnail" miniature of the full-resolution version;
of a size somewhere between that of a standard file icon (34 pixels
square) and a postage stamp (approx 70 x 85 pix).

Now it is possible to use the WIDTH and HEIGHT attributes of the IMG
tag to scale an image down to a smaller size;  but this is a ridiculous
way of doing it, because it requires the full-sized file to be (slowly)
fetched, just to display a titchy image which could have been held in
a considerably smaller file!

So the practical solution is to create a second (and much smaller) image
file on the site, which holds the thumbnail, and is used in the IMG tag
inside the <A></A> element, as in the example:

   <A HREF="large.jpeg"><IMG SRC="small.gif" ALT="Fetch picture"></A>

Since the thumbnail is a different file to the main image, it must have
a different name (or since recently, it could have the same name but be
in a different directory).

The choice of JPEG or GIF will be determined by the nature of the image
itself:
For the main large image, JPEG is usually best for scanned photographs,
or (conversion of) some ArtWorks files with a lot of colour graduations
(even better quality may be achieved by using TIFF:  you can never rely
on a browser being able to render them, but they're OK for downloads),
but GIF is often better for screenshots of a drawfile or a line drawing;
For the thumbnail image, much the same could apply,  but since the idea
is to minimise the filesize rather than maximise the quality, I usually
find that GIF is more suitable.

The final choice of filetype for each version of the image may influence
your choice of filenames:  in RiscOS it is not possible to have (in the
same directory) two files of different filetypes but identical filename;
but you can have two names which differ only in their "slash-extension".

So some possible RiscOS ADFS name choices for image file pairs might be:
Fullsized:  piccy/jpeg   large/jpeg  photos.pict3/gif
Thumbnail:  piccy/gif    small/jpeg  thumbs.pict3/gif
etc.

                          John Alldred, 29June97 <john@protovale.co.uk>
                     http://www.argonet.co.uk/users/protovale/john.html