It might be easier to get feed back if you host the image here at the Guild. This way there is a thumbnail of the map attached to the post, and people can click the thumb to see the full version. Some people (like me atm) are blocked from redirects to images off site.

When you are in a posting window, there is a Go Advanced button to the bottom right of that window. Clicking that will bring you to another text window with plenty more option, one of which is what looks like a paper clip. Clicking THAT will open up another window which will guide you through the uploading procedure.

I hope this helps and good luck with your map.