People ask me this a lot, and the honest answer is: it depends on what the image is for. Each format exists because the others failed at something. Here is the plain-English version.
JPG compresses by throwing away detail your eye barely notices. That is why a phone photo comes out at 3MB, not 30MB. The catch is every save loses a little more quality. If you keep re-saving a JPG to JPG you will watch it slowly turn to mush.
PNG keeps every pixel exactly as it is. That makes it perfect for logos, screenshots, and anything with text or sharp edges. It also supports transparency, which JPG cannot do at all. The downside is file size: a photo saved as PNG can be 5 to 10 times bigger than the same photo as JPG. If you have a PNG that is bloating your email attachment, converting it with our PNG to JPG converter will shrink it dramatically.
WebP does what JPG does but better: same quality at roughly 30% smaller file size. Google pushed it hard, and now every major browser supports it. For website images, it is the smart default. If your site is slow because of heavy images, running them through our image to WebP converter is the easiest win you will get all week.
Photo that will be printed or edited later: keep JPG. Graphic, logo, or screenshot: PNG. Anything going on a website: WebP. That covers 95% of cases.
If you are unsure what you have, just open the file and look at the extension. Then pick the resize tool to fix dimensions while you are at it.