A heat map (aka filled map) is essentially a set of filled polygons on a background. When Tableau does a filled geographical map, it is using built-in polygons, like the borders of states for example. Is your background image a geographic map, or are we talking about a building floor plan or similar? If it is a geographic map, Tableau's built-in functionality will still work. If it is a building floor plan, you will need coordinates to form polygons using the same coordinate system as the floor plan. These will need to be in a table with fields similar to:
[Polygon ID], [Point Order], [x], [y]
Does that help? If you need more information, can you attach a sample packaged workbook illustrating your challenge? See Anonymize your Tableau Package Data for Sharing |Tableau Support Community if you have proprietary data.