lingohaus I could manage to change the size of the map via editing /t3-assets/css/css-aa382-26435.css
You should not editing any file in /t3-assets/ folder as this is temporary folder which holds the optimized css/js files only. Instead, you copy your custom css change into this file:
/templates/ja_property/css/custom.css (Create this file if it doesn't exist) - this file will be not affected when you update template.
For the map, template uses JA Google map plugin to load the map there, I'm still not entirely sure about your idea of changing the map to picture here.