Short answer
Drop your JPG or PNG files into the free image to WebP converter on this site. It runs in your browser, so nothing is uploaded, and it gives back smaller WebP files you can resize and rename. Use quality 80 to 85 and a longest side of about twice the size the image shows at.
Free tools for this
Why WebP
WebP files are usually much smaller than JPG or PNG at the same visible quality, and every current browser shows them. Smaller product images mean a faster page on a phone, and speed is both a ranking input and a reason shoppers leave.
Resize first, then compress
The biggest saving is usually the dimensions, not the format. A 4,000 pixel photo shown at 800 pixels wide is wasting most of its bytes.
Resize to about twice the largest size it shows at, which keeps it sharp on high density phone screens, then convert.
Try it now: Image to WebP Converter → · Speed Report →
Quality settings
Quality 80 to 85 looks the same to most eyes at a fraction of the size. Go lower for backgrounds and thumbnails. Keep it higher for detailed close-ups where texture is the point.
Shopify does not do it all for you
Shopify serves WebP to browsers that ask for it. A client that does not ask, such as some crawlers and link preview tools, can be sent the original file.
In our own test on a client's store, a 5.3 MB PNG master was served as it was to a client that does not ask for WebP, against 177 KB for an optimised master. Upload an optimised image and every client gets a small one.
WordPress and WooCommerce
A plugin can convert images when you upload them, or you can convert them first and upload the WebP. Either way, resize before you upload, and keep the originals somewhere safe.
Name the file
The file name becomes the image address, and search engines read it. blue-cotton-shirt.webp says more than IMG_2043.webp. Use lowercase and hyphens.
Add alt text too, describing what the picture shows. The file name and the alt text are two separate fields and both matter.
What not to convert
Keep logos and icons as SVG, which stays sharp at any size. Keep your originals. Check one converted image against the original on a real phone before you convert a whole catalogue.