JPG is the format every upload form accepts, and for photographic PNGs it is dramatically smaller. The catch is transparency: JPG has none, so transparent pixels must be filled. This page opens the Image Converter pre-set to JPG with white as the fill; change it under Transparency to match where the image will be placed.
Screenshots and graphics with sharp text should use quality 85–90 — JPEG artefacts show first around hard edges. Photographs are fine at 80. If the destination accepts it, PNG to WebP gives smaller files and keeps transparency.
This is a pre-set view of the Image Converter. Every option in the toolbar can still be changed.