Many thanks for your help
Siteground do have a WebP option, but it is definitely disabled (and it’s not set as on as default)
So on the homepage, only the pencil drawing of the boy (second image, below the header) is being served as WebP. The rest are still Jpegs. And on this page for example https://www.annabregmanportraits.co.uk/photos/ the photos of the two girls are WebP and the picture of the dog is still a jpeg. On this page https://www.annabregmanportraits.co.uk/feedback/ the photo is WebP and the drawing is a jpeg.