Need some help converting an alpha picture to a heightmap in photoshop

[clyspe]

I'm not looking for geological accuracy, as the nature of my map is very fantastical (a giant plant based beast follows the equinox across the equator, his footsteps repels the land and I'm planning on adding broken land archipelagos around his path later in worldpainter.) However, I'd like to have some kind of ocean floor, having the land raise up to the continents. Is there a simple way to do this in photoshop? I tried playing around with blending options, but outer glow wasn't cooperating. I need the heightmap to feed into worldpainter, as I'm using it in Minecraft.

I know the colors need to be inverted for it to work as a heightmap, it's just easier for me to see this way. Thanks in advance for your help

edit: wanted people to know that I think I figured it out, ish. It looks like I'll just have to do it by hand in worldpainter but it isn't that big a deal since the most I want from the map is

1. a map (lol) of where countries are going to go so I can start getting into prewriting and

2. There for basic renders of the landscape (for huge establishing shots) and

3. For hand drawn maps (I'll have to follow some tutorials to get those done) that are hanging on walls and what not. I wish the search function was functional, but this seems like a pretty cool forums and I'll probably lurk for a while, even if I won't have much to say for a couple months while I'm building.