Some sites still don't support PNG lol. Usually it's places where they specifically expect a photo, where they'll hard code the validation to only check if it's a JPEG. I once had to upload a picture of my face for something and the original JPEG was too large, so I took a screenshot and tried uploading that but it wouldn't work because it only accepted JPEGs. So I had to figure out how to compress the image locally on my phone because I didn't want to upload a picture of me to one of those online converters.