What's the difference between Map Image and Map Tile?

The main key difference is that the rendering process. In map image everything happens at the server side(HERE) and in response the only image will be sent back, whereas in map tiles the row data in the JSON format is returned and the rendering of row JSON data happens at client side.

The other key difference is The Mercator Projection, The HERE Map Tile v2 serves map tiles obtained by mapping points on the surface of a sphere (the globe) to points on a plane, using the normalized Mercator projection. For other details please refer the below dev guide.

https://developer.here.com/documentation/map-tile/dev_guide/topics/quick-start-dhc.html

https://developer.here.com/documentation/map-image/dev_guide/topics/display-options.html