Earlier quoted context omitted.
Grab a copy of netpbm or GraphicsMagick that's compiled for Windows. Or, grab a free image editor ( https://www.getpaint.net/ is popular for Windows), open in one format, save in the other.
This. I honestly don't understand why people use online image converters when there are high-quality OSS programs that will do the job locally. I'll bet that the majority of the online converters are using these programs to do the actual conversion anyway.
I have optimage and imageoptim on MacOS too and they are good enough at this point.