The easiest way to import & resize properly an image to Roll20 is as follows:
- Upload your map image to Roll20 library
- Edit the Page settings to make sure the grid size is 70x70 px
- Update the page size to fit the size of the image. You can read the grid dimensions in the Export Image window in RPG Map. For example, if you export a 40x30 map in RPG Map, make sure the page is 40x30 too.
- Add your image to the page, and resize to fill the whole page.
The grid should match perfectly with Roll20.
Sorry, it's a bit complex, but that's the way Roll20 is :)