If you want fast, no-frills image conversions, then the Internet is probably the best place for it, as there are tons of nice conversion web apps.

Still, there are a lot of advantages to having a desktop/local image converter on your system. For starters, there's the issue of privacy. Then, desktop apps are usually more convenient to use (for multiple operations) and arguably faster than their web counterparts.

That said, if you want to convert images to .webp, then WebP Converter is worth checking out.

WebP Converter is a typical Linux Flatpak app that allows users to convert images to WebP format (arguably best suited for fast web loading).

It's a free and open-source app that works exactly as intended on Linux desktops. The GUI is fairly simplistic, but the cool thing about this app is that it also provides control over the whole conversion process.

For instance, it allows users to batch-convert images (it's worth noting that it works with all popular image formats such as PNG, JPG, GIF, and so forth) while still controlling the compression settings (from 1 to 100%).

As far as I can determine, a 75% compression ratio yields very good results, but the 60% compression ratio is also decent (with minimal reduction in quality and file sizes of around 90% of its original size).

WebP Converter is a very simple app, but please note that you still have to select the images (after they are loaded into the app) in order to actually begin the conversion process (signaled by the Convert button turning blue).

WebP Converter is a good app to have if you're converting images to WebP on a daily basis. The only real annoyance (and it's not a functional one) is the fact that I have no idea what's it based on. This is also due to the fact that the app only appears on Flathub and I am unable to find a proper GitHub repository.