Compress thumbnail images

[JackDunnCodes]

If the uploader sends a png convert it to a jpg (or maybe webp - all modern browsers support it, except of course Macs that can't do it on versions less than 11 big sur: https://caniuse.com/webp)

[SuperNinja-4965]

jpg giff and png are probably the best formats to ensure support: https://en.wikipedia.org/wiki/Comparison_of_web_browsers#Image_format_support