Yep, all you need is a .JPG image for the map. There's no actual "VTT format".

If you also wanted to create tokens, elements, etc., for use in VTT, those would need be in .PNG format with a transparent background.

Take a look at these videos from Roll20. They'll give you a better understanding of what VTTs are, how they work, what they can do, etc.
Roll20 GM Overview
New Player Overview
Roll20 -- Grid Alignment Tool