Well, let's not get carried away. If you use one of my skins, and you choose the jAlbum core setting to produce WEBP outputs, you get thumbnails, slide images, and folder images that are all WEBP.

The exception is the theme image, of course. Familiar territory. It's been a while since we ventured down that particular rabbit hole.

The most visible part of the degradation happens on the smooth gradients - like the clouds -, so I can not tell these apart from the face part.

Historically, jAlbum did not have WebP support, which came long after my skins had a separate folder and hero image generation.

In the first version, the WebP file size was dramatically smaller, but the visible blockiness was so strong that we had to ask the WebP library's author to refine the default settings. After that, the benefit of WebP vanished. Perhaps there could be better WebP libraries for Java. If you can find one, please give us a link.

As far as I can see, jAlbum still does not offer WebP generation for the skins. It saves images in JPEG or PNG, depending on the input image. (See the API doc) So, even if I had time for this development, I could not achieve this functionality. But I am working on a new skin that might attract more users. As we don't have infinite time, we must prioritize,

There is the set outputFormat() api call, https://jalbum.net/api/se/datadosen/jalbum/AlbumBean.html#setOutputFormat(net.jalbum.OutputFormat)

You can certainly read the output format in init, don’t know about setting it, not tried it. When it comes to the api, for me, it is still often trial and error.

For theme images I use method 2A: https://jalbum.net/help/en/Sample_scripts#Theme_images
That produces Webp theme images if Webp is the output format. But it does mean theme images are remade on every make album.

Unfortunately, I cannot use the theme image creator built-in jAlbum, as my skins offer more features: 3 different sizes, B&W, HiDpi.

Just tried an experiment using the system console:

// Code input area
AlbumObject[] selectedAos = JAlbumContext.getInstance().getExplorer().explorer.getSelectedAlbumObjects();
rif = selectedAos[0].getRepresentingImageFile();
if(rif != null) {
	AlbumImage ai = new AlbumImage(rif, engine);
	minDim = Math.min(ai.getBufferedImage().getWidth(), ai.getBufferedImage().getHeight());
	JAFilter sq = new CropFilter();
	sq.setBounds(new Dimension(minDim, minDim));
	ai = ai.applyFilter(sq);
	ai = ai.scaleToFit(new Dimension(400,400));
	ai.saveImage(new File(rootOutputDirectory, "folderthumb.webp"));
}

Selecting a single in a project the file produced is a webP file, not just named webp.

Just thought I'd mention it, as it seems to imply setting the name of a file is enough to generate that image type in multiple image methods.

That works, but it ignores the WebP quality setting, so use with caution!

ETA: It appears to default to 75%.