Why? I assume the intention is to show these images on a webpage somewhere. WebP is well-supported by browsers and can store lossless images at better compression ratios than PNG, so why not use it? I don't think using a lossy format like JPEG makes much sense. JPEG is a fine format for photos, but for HTML content rendered as an image I assume most people would want a lossless format so you don't get artifacts.

Definitely should be WebP.

Because it's impossible to use in other tools. Only browsers get it. But I agree about lossy images for text.