1) Create a new page, and makes sure the grid utility is on and set to 70x70, see below for an example.
2) Upload the map above to your Library. Make sure you are on the Map Layer (red circle on the image below), and then drag the map onto the Page. Notice how Roll20 by default shrinks the map, annoying, but ok.
3) Next you will need to right click on the map, move to Advanced, and then click on Set Dimensions.
4) Set the map image to 2100x2800 pixels. You may need to move the map depending upon how large you set the page size.
5) This particular map will fit in the default page set at 25x25 squares. However, after setting the dimensions in step 3 you will have to move the map to fit into the visible page (or resize the page under settings). I have done this in the image below.
6) You can see here the map lines up perfectly with the default grid settings.
Now, understand that this will work for this map and most future maps, my previous maps were not created using this template and thus will not work in this manner. I may go back and change the most recent ones I have done as I may be able to port them over to this new template. I will have to see.
AND finally, here are a couple different versions of the map used in this tutorial.