Start with the image type
Photos usually become smaller as JPG or WebP. Screenshots and graphics may stay clearer as PNG or WebP. AVIF can be very small, but compatibility is narrower.
Best first conversions
Try PNG to JPG when transparency is not needed. Try JPG to WebP for website images. Try PNG to WebP when you still want strong web quality with a smaller file.
Check the destination
Some upload forms accept only JPG or PNG. A smaller WebP or AVIF file is not useful if the website rejects the format.