JPEG is for photographs. It is lossy, has no transparency, and is the smallest sensible choice for anything with continuous tone. PNG is lossless and keeps an alpha channel, which makes it right for screenshots, diagrams, logos and anything with hard edges or text, and quite wrong for a holiday photo, where it will produce a file several times larger for no visible gain. WebP does both jobs and lands roughly 25 to 30% smaller than either, at the cost of a handful of older applications that still will not open it. TIFF and BMP exist here for the software that demands them.

Transparency is the thing that catches people

Converting PNG to JPEG has to do something about the transparent pixels, because JPEG has nowhere to put them. Left alone they typically come out black. The tools here flatten them onto white by default, which is what most people want and almost nobody expects to have to ask for. Going the other way, JPEG to PNG, does not create transparency and does not recover quality; it produces a larger file holding the same picture, artefacts included.

Resizing

Scaling down is safe and is usually the fastest way to make a file smaller, because it removes detail rather than corrupting it. Scaling up cannot invent detail that was never captured. The upscaler here uses Lanczos resampling, which is a good interpolation and still an interpolation: it will make a small image larger and smoother, not sharper. Set one dimension and leave the other at zero on any of these tools and the aspect ratio is preserved.