Thank you very much, flop25. Your information has helped me to succeed. :) I read the Piwigo documentation at the following three webpages:

http://piwigo.org/doc/doku.php?id=en:web-appaerance
http://piwigo.org/doc/doku.php?id=en:ca … management
http://piwigo.org/doc/doku.php?id=en:remote_site

I use FTP to upload images. I know that galleries should be uploaded to the "galleries" directory in the root level of the piwigo installation. I created a directory (folder) in the galleries folder for one specific gallery (I called it "Free_Wallpapers" (without the double quotes)). I then scaled (resized) the very large (1920x1200 pixel) images to 800x500 pixels on my local computer, and uploaded these scaled-down images to the "Free_Wallpapers" directory I had created inside the "galleries" directory.

Then I created a new directory inside "Free_Wallpapers" named "pwg_high" to contain the full high-definition photos, and I uploaded the original 1920x1200 pixel images to the pwg_high directory.

Then after logging into Piwigo at our website and going to the Piwigo administration webpage, I used the Thumbnails tool to create the small thumbnail images and synchronized the directories and photos so that Piwigo would scan for, recognize and catalog the directories I had created and photos I had uploaded.

After that, the gallery works exactly as I prefer and as you mentioned. In each gallery (category) the thumbnails are displayed. If a person clicks the thumbnail, he or she sees the mid-size images (the ones I scaled down to 800x500 pixels). And because of the existence of the pwg_high directory, an icon appears that a user may click to download the full-size, high-definition image, and a "Click on the photo to see it in high definition" message appears just below the mid-sized (web browser viewable size) image.

All works perfectly. Thank you so much for helping me. Best wishes--
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