The short answer
JPG for photos, PNG when you need transparency or sharp text/graphics, WEBP when you want the best of both and whatever you're using actually supports it. Here's the reasoning, if you want it.
JPG: built for photos
JPG's compression is basically tuned for photos: smooth gradients, tons of color variation, no hard edges anywhere. It gets away with a lot of compression on a photo because the artifacts it introduces look like the noise a photo already has. Point the same compression at a screenshot with flat colors and sharp text, though, and suddenly you can see it — fuzzy halos around letters, ugly banding in solid backgrounds.
It also just doesn't do transparency at all, so logos or anything meant to sit on top of other content are the wrong job for JPG.
PNG: built for graphics and transparency
PNG is lossless, so every pixel stays exactly as it was, and it supports a real transparent background. That's why it's the go-to for logos, icons, screenshots with text, anything that needs to sit on a see-through layer. The cost is size — a detailed photo saved as PNG can easily be several times bigger than the same photo as a JPG, since lossless encoding has no way to throw away the fine detail that lossy compression happily discards.
WEBP: the modern all-rounder
WEBP does both lossy and lossless in one format, handles transparency too, and usually beats JPG or PNG on file size for the same visual quality. Compatibility used to be its weak spot, but basically every current browser and OS reads it fine now. Really the only reason to still default to JPG or PNG is some specific tool, printer, or older system down the line that hasn't caught up to WEBP yet.
What this means for Image → PDF
Our Image → PDF tool takes all three and treats them the same once you convert — each one becomes a full page in the resulting PDF at its original quality. The format choice matters more before you get to that step. Photographing documents? JPG at a moderate quality keeps things reasonably sized. Capturing screenshots or graphics with actual text in them? PNG, or WEBP in lossless mode, keeps everything sharp.