It appears that html to jpg conversion might represent two things, neither of which are actually file conversion in proper sense.
You can create a picture of the output of .html page, by opening it in a web browser and using any screen capturing utility to produce a .jpg picture, which is basically the same thing that many html to picture (image) converters offer.
The option meaning of this conversion would be export of one of the images located on some .html website. This is only possible if you have access to the source files as the picture will only be referenced in the HTML document.