I need a basic service which converts image files into SVG files

We have a service that we provide to customers which depends on them providing us with a picture that must be in SVG format. However, most of our customers have PNG's, JPG's, JPEG's etc.

I need a basic service or even bit of code that when given an image file, will convert it to an SVG.

It does not need to be pretty or fancy. It can literally be a box which allows a file to be uploaded and spits out an SVG at the end.