depends on how detailed you want to be. If you want to build world maps from scratch, I would pop over to the world-building section of the homebrew forum. They can point you to a few good programs for that.

If it's for combat, and you don't want have to re-upload the link every time it's updated (like pyromancers), then you can try google docs. Just import an image (preferably with a grid), and give everyone permission to edit the doc. (just make sure they write down their actions in the PbP thread so you can keep track for everything).

Or, you can make it just a spreadsheet and color in the cells appropriately. Looks basic but it gets the job done.