I've been using Pyromancers.com for my maps, too.
I've found that exporting the map at a 70x70 grid makes it super simple to get the map to align to the grid.
After you upload your map to roll20, you should right-click on the map (it has probably shrunk down to 1x1 grid), select Advanced > Set Dimensions.
Now, back in Windows Explorer or Finder, you'll want to click on the jpg or png of the map so you can see what the dimensions of the map are. Then, enter those dimensions into the roll20 dialog that popped up when you selected "Advanced > Set Dimensions".
Once it gets set, you can drag the map up to the upper-left hand corner of the page. If you have snap-to-grid enabled, it should all work out.
You may need to resize your grid to match your map. Just divide the height by 70 to see how many vertical units you need. Then divide the width by 70 to see how many horizontal units you need.
Good luck!