I’ve been looking for software that does exactly this for a while:
An intuitive, detailed map-making program that works right in your web browser and allows export to jpg and png. It’s got a built-in grid for D&D goodness, and – although I haven’t tried it – a “scene-builder” designed specifically for PbP games.
I’m so pleased! PbP 4e D&D is now a possibility! 😀
I mean, it was before, but what with my workplace blocking all known image-hosting sites and Google Docs, my ‘post-from-work’ habit would have been untenable.